opinion: Justice Rehnquist
  delivered the opinion of the Court.
  Respondents Lance and Susan Gates were indicted for violation of state drug laws after police officers, executing a search warrant, discovered marihuana and other contraband in their automobile and home. Prior to trial the Gateses moved to suppress evidence seized during this search. The Illinois Supreme Court affirmed the decisions of lower state courts granting the motion. 85 Ill. 2d 376, 423 N. E. 2d 887 (1981). ■ It held that the affidavit submitted in support of the State’s application for a warrant to search the Gateses’ property was inadequate under this Court’s decisions in Aguilar v. Texas, 378 U. S. 108 (1964), and Spinelli v. United States, 393 U. S. 410 (1969).
  We granted certiorari to consider the application of the Fourth Amendment to a magistrate’s issuance of a search warrant on the basis of a partially corroborated anonymous informant’s tip. 454 U. S. 1140 (1982). After receiving briefs and hearing oral argument on this question, however, we requested the parties to address an additional question:
  “[Wjhether the rule requiring the exclusion at a criminal trial of evidence obtained in violation of the Fourth Amendment, Mapp v. Ohio, 367 U. S. 643 (1961); Weeks v. United States, 232 U. S. 383 (1914), should to any extent be modified, so as, for example, not to require the exclusion of evidence obtained in the reasonable belief that the search and seizure at issue was consistent with the Fourth Amendment.” 459 U. S. 1028 (1982).
  We decide today, with apologies to all, that the issue we framed for the parties was not presented to the Illinois courts and, accordingly, do not address it. Rather, we consider the question originally presented in the petition for certiorari, and conclude that the Illinois Supreme Court read the requirements of our Fourth Amendment decisions too restrictively. Initially, however, we set forth our reasons for not addressing the question regarding modification of the exclusionary rule framed in our order of November 29,1982. Ibid.
  HH
  Our certiorari jurisdiction over decisions from state courts derives from 28 U. S. C. § 1257, which provides that “[f]inal judgments or decrees rendered by the highest court of a State in which a decision could be had, may be reviewed by the Supreme Court as follows: ... (3) By writ of certiorari, . . . where any title, right, privilege or immunity is specially set up or claimed under the Constitution, treaties or statutes of... the United States.” The provision derives, albeit with important alterations, see, e. g., Act of Dec. 23, 1914, ch. 2, 38 Stat. 790; Act of June 25, 1948, § 1257, 62 Stat. 929, from the Judiciary Act of 1789, § 25, 1 Stat. 85.
  Although we have spoken frequently on the meaning of §1257 and its predecessors, our decisions are in some respects not entirely clear. We held early on that § 25 of the Judiciary Act of 1789 furnished us with no jurisdiction unless a federal question had been both raised and decided in the state court below. As Justice Story wrote in Crowell v. Randell, 10 Pet. 368, 392 (1836): “If both of these requirements do not appear on the record, the appellate jurisdiction fails.” See also Owings v. Norwood’s Lessee, 5 Cranch 344 (1809).
  More recently, in McGoldrick v. Compagnie Generale Transatlantique, 309 U. S. 430, 434-435 (1940), the Court observed:
  “But it is also the settled practice of this Court, in the exercise of its appellate jurisdiction, that it is only in exceptional cases, and then only in cases coming from the federal courts, that it considers questions urged by a petitioner or appellant not pressed or passed upon in the courts below.... In cases coming here from state courts in which a state statute is assailed as unconstitutional, there are reasons of peculiar force which should lead us to refrain from deciding questions not presented or decided in the highest court of the state whose judicial action we are called upon to review. Apart from the reluctance with which every court should proceed to set aside legislation as unconstitutional on grounds not properly presented, due regard for the appropriate relationship of this Court to state courts requires us to decline to consider and decide questions affecting the validity of state statutes not urged or considered there. It is for these reasons that this Court, where the constitutionality of a statute has been upheld in the state court, consistently refuses to consider any grounds of attack not raised or decided in that court.”
  Finally, the Court seemed to reaffirm the jurisdictional character of the rule against our deciding claims “not pressed nor passed upon” in state court in State Farm Mutual Automobile Ins. Co. v. Duel, 324 U. S. 154, 160 (1945), where we explained that “[sjince the [State] Supreme Court did not pass on the question, we may not do so.” See also Hill v. California, 401 U. S. 797, 805-806 (1971).
  Notwithstanding these decisions, however, several of our more recent cases have treated the so-called “not pressed or passed upon below” rule as merely a prudential restriction. In Terminiello v. Chicago, 337 U. S. 1 (1949), the Court reversed a state criminal conviction on a ground not urged in state court, nor even in this Court. Likewise, in Vachon v. New Hampshire, 414 U. S. 478 (1974), the Court summarily reversed a state criminal conviction on the ground, not raised in state court, or here, that it had been obtained in violation of the Due Process Clause of the Fourteenth Amendment. The Court indicated in a footnote, id., at 479, n. 3, that it possessed discretion to ignore the failure to raise in state court the question on which it decided the case.
  In addition to this lack of clarity as to the character of the “not pressed or passed upon below” rule, we have recognized that it often may be unclear whether the particular federal question presented in this Court was raised or passed upon below. In Dewey v. Des Moines, 173 U. S. 193, 197-198 (1899), the fullest treatment of the subject, the Court said that “[i]f the question were only an enlargement of the one mentioned in the assignment of errors, or if it were so connected with it in substance as to form but another ground or reason for alleging the invalidity of the [lower court’s] judgment, we should have no hesitation in holding the assignment sufficient to permit the question to be now raised and argued. Parties are not confined here to the same arguments which were advanced in the courts below upon a Federal question there discussed.” We have not attempted, and likely would not have been able, to draw a clear-cut line between cases involving only an “enlargement” of questions presented below and those involving entirely new questions.
  The application of these principles in the instant case is not entirely straightforward. It is clear in this case that respondents expressly raised, at every level of the Illinois judicial system, the claim that the Fourth Amendment had been violated by the actions of the Illinois police and that the evidence seized by the officers should be excluded from their trial. It also is clear that the State challenged, at every level of the Illinois court system, respondents’ claim that the substantive requirements of the Fourth Amendment had been violated. The State never, however, raised or addressed the question whether the federal exclusionary rule should be modified in any respect, and none of the opinions of the Illinois courts give any indication that the question was considered.
  The case, of course, is before us on the State’s petition for a writ of certiorari. Since the Act of Dec. 23, 1914, ch. 2, 38 Stat. 790, jurisdiction has been vested in this Court to review state-court decisions even when a claimed federal right has been upheld. Our prior decisions interpreting the “not pressed or passed on below” rule have not, however, involved a State’s failure to raise a defense to a federal right or remedy asserted below. As explained below, however, we can see no reason to treat the State’s failure to have challenged an asserted federal claim differently from the failure of the proponent of a federal claim to have raised that claim.
  We have identified several purposes underlying the “not pressed or passed upon” rule: for the most part, these are as applicable to the State’s failure to have opposed the assertion of a particular federal right, as to a party’s failure to have asserted the claim. First, “[questions not raised below are those on which the record is very likely to be inadequate since it certainly was not compiled with those questions in mind.” Cardinale v. Louisiana, 394 U. S. 437, 439 (1969). Exactly the same difficulty exists when the State urges modification of an existing constitutional right or accompanying remedy. Here, for example, the record contains little, if anything, regarding the subjective good faith of the police officers that searched the Gateses’ property — which might well be an important consideration in determining whether to fashion a good-faith exception to the exclusionary rule. Our consideration of whether to modify the exclusionary rule plainly would benefit from a record containing such facts.
  Likewise, “due regard for the appropriate relationship of this Court to state courts,” McGoldrick v. Compagnie Generale Transatlantique, 309 U. S., at 434-435, demands that those courts be given an opportunity to consider the constitutionality of the actions of state officials, and, equally important, proposed changes in existing remedies for unconstitutional actions. Finally, by requiring that the State first argue to the state courts that the federal exclusionary rule should be modified, we permit a state court, even if it agrees with the State as a matter of federal law, to rest its decision on an adequate and independent state ground. See Cardinale, supra, at 439. Illinois, for example, adopted an exclusionary rule as early as 1923, see People v. Brocamp, 307 Ill. 448, 138 N. E. 728 (1923), and might adhere to its view even if it thought we would conclude that the federal rule should be modified. In short, the reasons supporting our refusal to hear federal claims not raised in state court apply with equal force to the State’s failure to challenge the availability of a well-settled federal remedy. Whether the “not pressed or passed upon below” rule is jurisdictional, as our earlier decisions indicate, see supra, at 217-219, or prudential, as several of our later decisions assume, or whether its character might be different in cases like this from its character elsewhere, we need not decide. Whatever the character of the rule may be, consideration of the question presented in our order of November 29, 1982, would be contrary to the sound justifications for the “not pressed or passed upon below” rule, and we thus decide not to pass on the issue.
  The fact that the Illinois courts affirmatively applied the federal exclusionary rule — suppressing evidence against respondents — does not affect our conclusion. In Morrison v. Watson, 154 U. S. 111 (1894), the Court was asked to consider whether a state statute impaired the plaintiff in error’s contract with the defendant in error. It declined to hear the case because the question presented here had not been pressed or passed on below. The Court acknowledged that the lower court’s opinion had restated the conclusion, set forth in an earlier decision of that court, that the state statute did not impermissibly impair contractual obligations. Nonetheless, it held that there was no showing that “there was any real contest at any stage of this case upon the point,” id., at 115, and that without such a contest, the routine restatement and application of settled law by an appellate court did not satisfy the “not pressed or passed upon below” rule. Similarly, in the present case, although the Illinois courts applied the federal exclusionary rule, there was never “any real contest” upon the point. The application of the exclusionary rule was merely a routine act, once a violation of the Fourth Amendment had been found, and not the considered judgment of the Illinois courts on the question whether application of a modified rule would be warranted on the facts of this case. In such circumstances, absent the adversarial dispute necessary to apprise the state court of the arguments for not applying the exclusionary rule, we will not consider the question whether the exclusionary rule should be modified.
  Likewise, we do not believe that the State’s repeated opposition to respondents’ substantive Fourth Amendment claims suffices to have raised the question whether the exclusionary rule should be modified. The exclusionary rule is “a judicially created remedy designed to safeguard Fourth Amendment rights generally” and not “a personal constitutional right of the party aggrieved.” United States v. Calandra, 414 U. S. 338, 348 (1974). The question whether the exclusionary rule’s remedy is appropriate in a particular context has long been regardéd as an issue separate from the question whether the Fourth Amendment rights of the party seeking to invoke the rule were violated by police conduct. See, e. g., United States v. Havens, 446 U. S. 620 (1980); United States v. Ceccolini, 435 U. S. 268 (1978); United States v. Calandra, supra; Stone v. Powell, 428 U. S. 465 (1976). Because of this distinction, we cannot say that modification or abolition of the exclusionary rule is “so connected with [the substantive Fourth Amendment right at issue] as to form but another ground or reason for alleging the invalidity” of the judgment. Dewey v. Des Moines, 173 U. S., at 197-198. Rather, the rule’s modification was, for purposes of the “not pressed or passed upon below” rule, a separate claim that had to be specifically presented to the state courts.
  Finally, weighty prudential considerations militate against our considering the question presented in our order of November 29, 1982. The extent of the continued vitality of the rules that have developed from our decisions in Weeks v. United States, 232 U. S. 383 (1914), and Mapp v. Ohio, 367 U. S. 643 (1961), is an issue of unusual significance. Sufficient evidence of this lies just in the comments on the issue that Members of this Court recently have made, e. g., Bivens v. Six Unknown Fed. Narcotics Agents, 403 U. S. 388, 415 (1971) (Burger, C. J., dissenting); Coolidge v. New Hampshire, 403 U. S. 443, 490 (1971) (Harlan, J., concurring); id., at 502 (Black, J., dissenting); Stone v. Powell, supra, at 537-539 (White, J., dissenting); Brewer v. Williams, 430 U. S. 387, 413-414 (1977) (Powell, J., concurring); Robbins v. California, 453 U. S. 420, 437, 443-444 (1981) (Rehnquist, J., dissenting). Where difficult issues of great public importance are involved, there are strong reasons to adhere scrupulously to the customary limitations on our discretion. By doing so we “promote respect... for the Court’s adjudicatory process [and] the stability of [our] decisions.” Mapp v. Ohio, 367 U. S., at 677 (Harlan, J., dissenting). Moreover, fidelity to the rule guarantees that a factual record will be available to us, thereby discouraging the framing of broad rules, seemingly sensible on one set of facts, which may prove ill-considered in other circumstances. In Justice Harlan’s words, adherence to the rule lessens the threat of “untoward practical ramifications,” id., at 676 (dissenting opinion), not foreseen at the time of decision. The public importance of our decisions in Weeks and Mapp and the emotions engendered by the debate surrounding these decisions counsel that we meticulously observe our customary procedural rules. By following this course, we promote respect for the procedures by which our decisions are rendered, as well as confidence in the stability of prior decisions. A wise exercise of the powers confided in this Court dictates that we reserve for another day the question whether the exclusionary rule should be modified.
  l-H H — (
  We now turn to the question presented in the State’s original petition for certiorari, which requires us to decide whether respondents’ rights under the Fourth and Fourteenth Amendments were violated by the search of their car and house. A chronological statement of events usefully introduces the issues at stake. Bloomingdale, Ill., is a suburb of Chicago located in Du Page County. On May 3, 1978, the Bloomingdale Police Department received by mail an anonymous handwritten letter which read as follows:
  “This letter is to inform you that you have a couple in your town who strictly make their living on selling drugs. They are Sue and Lance Gates, they live on Greenway, off Bloomingdale Rd. in the condominiums. Most of their buys are done in Florida. Sue his wife drives their car to Florida, where she leaves it to be loaded up with drugs, then Lance flys down and drives it back. Sue flys back after she drops the car off in Florida. May 3 she is driving down there again and Lance will be flying down in a few days to drive it back. At the time Lance drives the car back he has the trunk loaded with over $100,000.00 in drugs. Presently they have over $100,000.00 worth of drugs in their basement.
  “They brag about the fact they never have to work, and make their entire living on pushers.
  “I guarantee if you watch them carefully you will make a big catch. They are friends with some big drugs dealers, who visit their house often.
  “Lance & Susan Gates
  “Greenway
  “in Condominiums”
  The letter was referred by the Chief of Police of the Bloomingdale Police Department to Detective Mader, who decided to pursue the tip. Mader learned, from the office of the Illinois Secretary of State, that an Illinois driver’s license had been issued to one Lance Gates, residing at a stated address in Bloomingdale. He contacted a confidential informant, whose examination of certain financial records revealed a more recent address for the Gateses, and he also learned from a police officer assigned to O'Hare Airport that “L. Gates” had made a reservation on Eastern Airlines Flight 245 to West Palm Beach, Fla., scheduled to depart from Chicago on May 5 at 4:15 p. m.
  Mader then made arrangements with an agent of the Drug Enforcement Administration for surveillance of the May 5 Eastern Airlines flight. The agent later reported to Mader that Gates had boarded the flight, and that federal agents in Florida had observed him arrive in West Palm Beach and take a taxi to the nearby Holiday Inn. They also reported that Gates went to a room registered to one Susan Gates and that, at 7 o’clock the next morning, Gates and an unidentified woman left the motel in a Mercury bearing Illinois license plates and drove northbound on an interstate highway frequently used by travelers to the Chicago area. In addition, the DEA agent informed Mader that the license plate number on the Mercury was registered to a Hornet station wagon owned by Gates. The agent also advised Mader that the driving time between West Palm Beach and Bloomingdale was approximately 22 to 24 hours.
  Mader signed an affidavit setting forth the foregoing facts, and submitted it to a judge of the Circuit Court of Du Page County, together with a copy of the anonymous letter. The judge of that court thereupon issued a search warrant for the Gateses' residence and for their automobile. The judge, in deciding to issue the warrant, could have determined that the modus operandi of the Gateses had been substantially corroborated. As the anonymous letter predicted, Lance Gates had flown from Chicago to West Palm Beach late in the afternoon of May 5th, had checked into a hotel room registered in the name of his wife, and, at 7 o’clock the following morning, had headed north, accompanied by an unidentified woman, out of West Palm Beach on an interstate highway used by travelers from South Florida to Chicago in an automobile bearing a license plate issued to him.
  At 5:15 a. m. on March 7, only 36 hours after he had flown out of Chicago, Lance Gates, and his wife, returned to their home in Bloomingdale, driving the car in which they had left West Palm Beach some 22 hours earlier. The Bloomingdale police were awaiting them, searched the trunk of the Mercury, and uncovered approximately 350 pounds of marihuana. A search of the Gateses’ home revealed marihuana, weapons, and other contraband. The Illinois Circuit Court ordered suppression of all these items, on the ground that the affidavit submitted to the Circuit Judge failed to support the necessary determination of probable cause to believe that the Gateses’ automobile and home contained the contraband in question. This decision was affirmed in turn by the Illinois Appellate Court, 82 Ill. App. 3d 749, 403 N. E. 2d 77 (1980), and by a divided vote of the Supreme Court of Illinois. 85 Ill. 2d 376, 423 N. E. 2d 887 (1981).
  The Illinois Supreme Court concluded — and we are inclined to agree — that, standing alone, the anonymous letter sent to the Bloomingdale Police Department would not provide the basis for a magistrate’s determination that there was probable cause to believe contraband would be found in the Gateses’ car and home. The letter provides virtually nothing from which one might conclude that its author is either honest or his information reliable; likewise, the letter gives absolutely no indication of the basis for the writer’s predictions regarding the Gateses’ criminal activities. Something more was required, then, before a magistrate could conclude that there was probable cause to believe that contraband would be found in the Gateses’ home and car. See Aguilar v. Texas, 378 U. S., at 109, n. 1; Nathanson v. United States, 290 U. S. 41 (1933).
  The Illinois Supreme Court also properly recognized that Detective Mader’s affidavit might be capable of supplementing the anonymous letter with information sufficient to permit a determination of probable cause. See Whiteley v. Warden, 401 U. S. 560, 567 (1971). In holding that the affidavit in fact did not contain sufficient additional information to sustain a determination of probable cause, the Illinois court applied a “two-pronged test,” derived from our decision in Spinelli v. United States, 393 U. S. 410 (1969). The Illinois Supreme Court, like some others, apparently understood Spinelli as requiring that the anonymous letter satisfy each of two independent requirements before it could be relied on. 85 Ill. 2d, at 383, 423 N. E. 2d, at 890. According to this view, the letter, as supplemented by Mader’s affidavit, first had to adequately reveal the “basis of knowledge” of the letterwriter — the particular means by which he came by the information given in his report. Second, it had to provide facts sufficiently establishing either the “veracity” of the affiant’s informant, or, alternatively, the “reliability” of the informant’s report in this particular case.
  The Illinois court, alluding to an elaborate set of legal rules that have developed among various lower courts to enforce the “two-pronged test,” found that the test had not been satisfied. First, the “veracity” prong was not satisfied because, “[t]here was simply no basis [for] concluding] that the anonymous person [who wrote the letter to the Bloomingdale Police Department] was credible.” Id., at 385, 423 N. E. 2d, at 891. The court indicated that corroboration by police of details contained in the letter might never satisfy the “veracity” prong, and in any event, could not do so if, as in the present case, only “innocent” details are corroborated. Id., at 390, 423 N. E. 2d, at 893. In addition, the letter gave no indication of the basis of its writer’s knowledge of the Gateses’ activities. The Illinois court understood Spinelli as permitting the detail contained in a tip to be used to infer that the informant had a reliable basis for his statements, but it thought that the anonymous letter failed to provide sufficient detail to permit such an inference. Thus, it concluded that no showing of probable cause had been made.
  We agree with the Illinois Supreme Court that an informant’s “veracity,” “reliability,” and “basis of knowledge” are all highly relevant in determining the value of his report. We do not agree, however, that these elements should be understood as entirely separate and independent requirements to be rigidly exacted in every case, which the opinion of the Supreme Court of Illinois would imply. Rather, as detailed below, they should be understood simply as closely intertwined issues that may usefully illuminate the commonsense, practical question whether there is “probable cause” to believe that contraband or evidence is located in a particular place.
  Ill
  This totality-of-the-circumstances approach is far more consistent with our prior treatment of probable cause than is any rigid demand that specific “tests” be satisfied by every informant’s tip. Perhaps the central teaching of our decisions bearing on the probable-cause standard is that it is a “practical, nontechnical conception.” Brinegar v. United States, 338 U. S. 160, 176 (1949). “In dealing with probable cause, ... as the very name implies, we deal with probabilities. These are not technical; they are the factual and practical considerations of everyday life on which reasonable and prudent men, not legal technicians, act.” Id., at 175. Our observation in United States v. Cortez, 449 U. S. 411, 418 (1981), regarding “particularized suspicion,” is also applicable to the probable-cause standard:
  “The process does not deal with hard certainties, but with probabilities. Long before the law of probabilities was articulated as such, practical people formulated certain common-sense conclusions about human behavior; jurors as factfinders are permitted to do the same — and so are law enforcement officers. Finally, the evidence thus collected must be seen and weighed not in terms of library analysis by scholars, but as understood by those versed in the field of law enforcement.”
  As these comments illustrate, probable cause is a fluid concept — turning on the assessment of probabilities in particular factual contexts — not readily, or even usefully, reduced to a neat set of legal rules. Informants’ tips doubtless come in many shapes and sizes from many different types of persons. As we said in Adams v. Williams, 407 U. S. 143, 147 (1972): “Informants’ tips, like all other clues and evidence coming to a policeman on the scene, may vary greatly in their value and reliability.” Rigid legal rules are ill-suited to an area of such diversity. “One simple rule will not cover every situation.” Ibid.
  Moreover, the “two-pronged test” directs analysis into two largely independent channels — the informant’s “veracity” or “reliability” and his “basis of knowledge.” See nn. 4 and 5, supra. There are persuasive arguments against according these two elements such independent status. Instead, they are better understood as relevant considerations in the totality-of-the-circumstances analysis that traditionally has guided probable-cause determinations: a deficiency in one may be compensated for, in determining the overall reliability of a tip, by a strong showing as to the other, or by some other indicia of reliability. See, e. g., Adams v. Williams, supra, at 146-147; United States v. Harris, 403 U. S. 573 (1971).
  If, for example, a particular informant is known for the unusual reliability of his predictions of certain types of criminal activities in a locality, his failure, in a particular case, to thoroughly set forth the basis of his knowledge surely should not serve as an absolute bar to a finding of probable cause based on his tip. See United States v. Sellers, 483 F. 2d 37 (CA5 1973). Likewise, if an unquestionably honest citizen comes forward with a report of criminal activity — which if fabricated would subject him to criminal liability — we have found rigorous scrutiny of the basis of his knowledge unnecessary. Adams v. Williams, supra. Conversely, even if we entertain some doubt as to an informant’s motives, his explicit and detailed description of alleged wrongdoing, along with a statement that the event was observed firsthand, entitles his tip to greater weight than might otherwise be the case. Unlike a totality-of-the-circumstances analysis, which permits a balanced assessment of the relative weights of all the various indicia of reliability (and unreliability) attending an informant’s tip, the “two-pronged test” has encouraged an excessively technical dissection of informants’ tips, with undue attention being focused on isolated issues that cannot sensibly be divorced from the other facts presented to the magistrate.
  As early as Locke v. United States, 7 Cranch 339, 348 (1813), Chief Justice Marshall observed, in a closely related context: “[T]he term ‘probable cause,’ according to its usual acceptation, means less than evidence which would justify condemnation .... It imports a seizure made under circumstances which warrant suspicion.” More recently, we said that “the quanta ... of proof” appropriate in ordinary judicial proceedings are inapplicable to the decision to issue a warrant. Brinegar, 338 U. S., at 173. Finely tuned standards such as proof beyond a reasonable doubt or by a preponderance of the evidence, useful in formal trials, have no place in the magistrate’s decision. While an effort to fix some general, numerically precise degree of certainty corresponding to “probable cause” may not be helpful, it is clear that “only the probability, and not a prima facie showing, of criminal activity is the standard of probable cause.” Spinelli, 393 U. S., at 419. See Model Code of Pre-Arraignment Procedure §210.1(7) (Prop. Off. Draft 1972); 1 W. LaFave, Search and Seizure § 3.2(e) (1978).
  We also have recognized that affidavits “are normally drafted by nonlawyers in the midst and haste of a criminal investigation. Technical requirements of elaborate specificity once exacted under common law pleadings have no proper place in this area.” United States v. Ventresca, 380 U. S. 102, 108 (1965). Likewise, search and arrest warrants long have been issued by persons who are neither lawyers nor judges, and who certainly do not remain abreast of each judicial refinement of the nature of “probable cause.” See Shadwick v. City of Tampa, 407 U. S. 345, 348-350 (1972). The rigorous inquiry into the Spinelli prongs and the complex superstructure of evidentiary and analytical rules that some have seen implicit in our Spinelli decision, cannot be reconciled with the fact that many warrants are — quite properly, 407 U. S., at 348-350 — issued on the basis of nontechnical, common-sense judgments of laymen applying a standard less demanding than those used in more formal legal proceedings. Likewise, given the informal, often hurried context in which it must be applied, the “built-in subtleties,” Stanley v. State, 19 Md. App. 507, 528, 313 A. 2d 847, 860 (1974), of the “two-pronged test” are particularly unlikely to assist magistrates in determining probable cause.
  Similarly, we have repeatedly said that after-the-fact scrutiny by courts of the sufficiency of an affidavit should not take the form of de novo review. A magistrate's “determination of probable cause should be paid great deference by reviewing courts.” Spinelli, supra, at 419. “A grudging or negative attitude by reviewing courts toward warrants,” Ventresca, 380 U. S., at 108, is inconsistent with the Fourth Amendment’s strong preference for searches conducted pursuant to a warrant; “courts should not invalidate warrants] by interpreting affidavits] in a hypertechnical, rather than a commonsense, manner.” Id., at 109.
  If the affidavits submitted by police officers are subjected to the type of scrutiny some courts have deemed appropriate, police might well resort to warrantless searches, with the hope of relying on consent or some other exception to the Warrant Clause that might develop at the time of the search. In addition, the possession of a warrant by officers conducting an arrest or search greatly reduces the perception of unlawful or intrusive police conduct, by assuring “the individual whose property is searched or seized of the lawful authority of the executing officer, his need to search, and the limits of his power to search.” United States v. Chadwick, 433 U. S. 1, 9 (1977). Reflecting this preference for the warrant process, the traditional standard for review of an issuing magistrate’s probable-cause determination has been that so long as the magistrate had a “substantial basis for . . . concluding]” that a search would uncover evidence of wrongdoing, the Fourth Amendment requires no more. Jones v. United States, 362 U. S. 257, 271 (1960). See United States v. Harris, 403 U. S., at 577-583. We think reaffirmation of this standard better serves the purpose of encouraging recourse to the warrant procedure and is more consistent with our traditional deference to the probable-cause determinations of magistrates than is the “two-pronged test.”
  Finally, the direction taken by decisions following Spinelli poorly serves “[t]he most basic function of any government”: “to provide for the security of the individual and of his property.” Miranda v. Arizona, 384 U. S. 436, 539 (1966) (White, J., dissenting). The strictures that inevitably accompany the “two-pronged test” cannot avoid seriously impeding the task of law enforcement, see, e. g., n. 9, supra. If, as the Illinois Supreme Court apparently thought, that test must be rigorously applied in every case, anonymous tips would be of greatly diminished value in police work. Ordinary citizens, like ordinary witnesses, see Advisory Committee’s Notes on Fed. Rule Evid. 701, 28 U. S. C. App., p. 570, generally do not provide extensive recitations of the basis of their everyday observations. Likewise, as the Illinois Supreme Court observed in this case, the veracity of persons supplying anonymous tips is by hypothesis largely unknown, and unknowable. As a result, anonymous tips seldom could survive a rigorous application of either of the Spinelli prongs. Yet, such tips, particularly when supplemented by independent police investigation, frequently contribute to the solution of otherwise “perfect crimes.” While a conscientious assessment of the basis for crediting such tips is required by the Fourth Amendment, a standard that leaves virtually no place for anonymous citizen informants is not.
  For all these reasons, we conclude that it is wiser to abandon the “two-pronged test” established by our decisions in Aguilar and Spinelli. In its place we reaffirm the totality-of-the-circumstances analysis that traditionally has informed probable-cause determinations. See Jones v. United States, supra; United States v. Ventresca, 380 U. S. 102 (1965); Brinegar v. United States, 338 U. S. 160 (1949). The task of the issuing magistrate is simply to make a practical, commonsense decision whether, given all the circumstances set forth in the affidavit before him, including the “veracity” and “basis of knowledge” of persons supplying hearsay information, there is a fair probability that contraband or evidence of a crime will be found in a particular place. And the duty of a reviewing court is simply to ensure that the magistrate had a “substantial basis for . . . concluding]” that probable cause existed. Jones v. United States, 362 U. S., at 271. We are convinced that this flexible, easily applied standard will better achieve the accommodation of public and private interests that the Fourth Amendment requires than does the approach that has developed from Aguilar and Spinelli.
  Our earlier cases illustrate the limits beyond which a magistrate may not venture in issuing a warrant. A sworn statement of an affiant that “he has cause to suspect and does believe” that liquor illegally brought into the United States is located on certain premises will not do. Nathanson v. United States, 290 U. S. 41 (1933). An affidavit must provide the magistrate with a substantial basis for determining the existence of probable cause, and the wholly conclusory statement at issue in Nathanson failed to meet this requirement. An officer’s statement that “[a]ffiants have received reliable information from a credible person and do believe” that heroin is stored in a home, is likewise inadequate. Aguilar v. Texas, 378 U. S. 108 (1964). As in Nathanson, this is a mere conclusory statement that gives the magistrate virtually no basis at all for making a judgment regarding probable cause. Sufficient information must be presented to the magistrate to allow that official to determine probable cause; his action cannot be a mere ratification of the bare conclusions of others. In order to ensure that such an abdication of the magistrate’s duty does not occur, courts must continue to conscientiously review the sufficiency of affidavits on which warrants are issued. But when we move beyond the “bare bones” affidavits present in cases such as Nathanson and Aguilar, this area simply does not lend itself to a prescribed set of rules, like that which had developed from Spinelli. Instead, the flexible, common-sense standard articulated in Jones, Ventresca, and Brinegar better serves the purposes of the Fourth Amendment’s probable-cause requirement.
  Justice Brennan’s dissent suggests in several places that the approach we take today somehow downgrades the role of the neutral magistrate, because Aguilar and Spinelli “preserve the role of magistrates as independent arbiters of probable cause . . . Post, at 287. Quite the contrary, we believe, is the case. The essential protection of the warrant requirement of the Fourth Amendment, as stated in Johnson v. United States, 333 U. S. 10 (1948), is in “requiring that [the usual inferences which reasonable men draw from evidence] be drawn by a neutral and detached magistrate instead of being judged by the officer engaged in the often competitive enterprise of ferreting out crime.” Id., at 13-14. Nothing in our opinion in any way lessens the authority of the magistrate to draw such reasonable inferences as he will from the material supplied to him by applicants for a warrant; indeed, he is freer than under the regime of Aguilar and Spinelli to draw such inferences, or to refuse to draw them if he is so minded.
  The real gist of Justice Brennan’s criticism seems to be a second argument, somewhat at odds with the first, that magistrates should be restricted in their authority to make probable-cause determinations by the standards laid down in Aguilar and Spinelli, and that such findings “should not be authorized unless there is some assurance that the information on which they are based has been obtained in a reliable way by an honest or credible person.” Post, at 283. However, under our opinion magistrates remain perfectly free to exact such assurances as they deem necessary, as well as those required by this opinion, in making probable-cause determinations. Justice Brennan would apparently prefer that magistrates be restricted in their findings of probable cause by the development of an elaborate body of case law dealing with the “veracity” prong of the Spinelli test, which in turn is broken down into two “spurs” — the informant’s “credibility” and the “reliability” of his information, together with the “basis of knowledge” prong of the Spinelli test. See n. 4, supra. That such a labyrinthine body of judicial refinement bears any relationship to familiar definitions of probable cause is hard to imagine. As previously noted, probable cause deals “with probabilities. These are not technical; they are the factual and practical considerations of everyday life on which reasonable and prudent men, not legal technicians, act "Brinegar v. United States, 338 U. S., at 175.
  Justice Brennan’s dissent also suggests that “[w]ords such as ‘practical,’ ‘nontechnical,’ and ‘common sense,’ as used in the Court’s opinion, are but code words for an overly permissive attitude towards police practices in derogation of the rights secured by the Fourth Amendment.” Post, at 290. An easy, but not a complete, answer to this rather florid statement would be that nothing we know about Justice Rutledge suggests that he would have used the words he chose in Brinegar in such a manner. More fundamentally, no one doubts that “under our Constitution only measures consistent with the Fourth Amendment may be employed by government to cure [the horrors of drug trafficking],” post, at 290; but this agreement does not advance the inquiry as to which measures are, and which measures are not, consistent with the Fourth Amendment. “Fidelity” to the commands of the Constitution suggests balanced judgment rather than exhortation. The highest “fidelity” is not achieved by the judge who instinctively goes furthest in upholding even the most bizarre claim of individual constitutional rights, any more than it is achieved by a judge who instinctively goes furthest in accepting the most restrictive claims of governmental authorities. The task of this Court, as of other courts, is to “hold the balance true,” and we think we have done that in this case.
  IV
  Our decisions applying the totality-of-the-circumstances analysis outlined above have consistently recognized the value of corroboration of details of an informant’s tip by independent police work. In Jones v. United States, 362 U. S., at 269, we held that an affidavit relying on hearsay “is not to be deemed insufficient on that score, so long as a substantial basis for crediting the hearsay is presented.” We went on to say that even in making a warrantless arrest an officer “may rely upon information received through an informant, rather than upon his direct observations, so long as the informant’s statement is reasonably corroborated by other matters within the officer’s knowledge.” Ibid. Likewise, we recognized the probative value of corroborative efforts of police officials in Aguilar — the source of the “two-pronged test” — by observing that if the police had made some effort to corroborate the informant’s report at issue, “an entirely different case” would have been presented. Aguilar, 378 U. S., at 109, n. 1.
  Our decision in Draper v. United States, 358 U. S. 307 (1959), however, is the classic case on the value of corroborative efforts of police officials. There, an informant named Hereford reported that Draper would arrive in Denver on a train from Chicago on one of two days, and that he would be carrying a quantity of heroin. The informant also supplied a fairly detailed physical description of Draper, and predicted that he would be wearing a light colored raincoat, brown slacks, and black shoes, and would be walking “real fast.” Id., at 309. Hereford gave no indication of the basis for his information.
  On one of the stated dates police officers observed a man matching this description exit a train arriving from Chicago; his attire and luggage matched Hereford’s report and he was walking rapidly. We explained in Draper that, by this point in his investigation, the arresting officer “had personally verified every facet of the information given him by Hereford except whether petitioner had accomplished his mission and had the three ounces of heroin on his person or in his bag. And surely, with every other bit of Hereford’s information being thus personally verified, [the officer] had ‘reasonable grounds’ to believe that the remaining unverified bit of Hereford’s information — that Draper would have the heroin with him — was likewise true,” id., at 313.
  The showing of probable cause in the present case was fully as compelling as that in Draper. Even standing alone, the facts obtained through the independent investigation of Mader and the DEA at least suggested that the Gateses were involved in drug trafficking. In addition to being a popular vacation site, Florida is well known as a source of narcotics and other illegal drugs. See United States v. Mendenhall, 446 U. S. 544, 562 (1980) (Powell, J., concurring in part and concurring in judgment); DEA, Narcotics Intelligence Estimate, The Supply of Drugs to the U. S. Illicit Market From Foreign and Domestic Sources in 1980, pp. 8-9. Lance Gates’ flight to West Palm Beach, his brief, overnight stay in a motel, and apparent immediate return north to Chicago in the family car, conveniently awaiting him in West Palm Beach, is as suggestive of a prearranged drug run, as it is of an ordinary vacation trip.
  In addition, the judge could rely on the anonymous letter, which had been corroborated in major part by Mader's efforts — just as had occurred in Draper. The Supreme Court of Illinois reasoned that Draper involved an informant who had given reliable information on previous occasions, while the honesty and reliability of the anonymous informant in this case were unknown to the Bloomingdale police. While this distinction might be an apt one at the time the Police Department received the anonymous letter, it became far less significant after Mader’s independent investigative work occurred. The corroboration of the letter’s predictions that the Gateses’ car would be in Florida, that Lance Gates would fly to Florida in the next day or so, and that he would drive the car north toward Bloomingdale all indicated, albeit not with certainty, that the informant’s other assertions also were true. “[Bjecause an informant is right about some things, he is more probably right about other facts,” Spinelli, 393 U. S., at 427 (White, J., concurring) — including the claim regarding the Gateses’ illegal activity. This may well not be the type of “reliability” or “veracity” necessary to satisfy some views of the “veracity prong” of Spinelli, but we think it suffices for the practical, common-sense judgment called for in making a probable-cause determination. It is enough, for purposes of assessing probable cause, that “[corroboration through other sources of information reduced the chances of a reckless or prevaricating tale,” thus providing “a substantial basis for crediting the hearsay.” Jones v. United States, 362 U. S., at 269, 271.
  Finally, the anonymous letter contained a range of details relating not just to easily obtained facts and conditions existing at the time of the tip, but to future actions of third parties ordinarily not easily predicted. The letterwriter’s accurate information as to the travel plans of each of the Gateses was of a character likely obtained only from the Gateses themselves, or from someone familiar with their not entirely ordinary travel plans. If the informant had access to accurate information of this type a magistrate could properly conclude that it was not unlikely that he also had access to reliable information of the Gateses’ alleged illegal activities. Of course, the Gateses’ travel plans might have been learned from a talkative neighbor or travel agent; under the “two-pronged test” developed from Spinelli, the character of the details in the anonymous letter might well not permit a sufficiently clear inference regarding the letterwriter’s “basis of knowledge.” But, as discussed previously, supra, at 235, probable cause does not demand the certainty we associate with formal trials. It is enough that there was a fair probability that the writer of the anonymous letter had obtained his entire story either from the Gateses or someone they trusted. And corroboration of major portions of the letter’s predictions provides just this probability. It is apparent, therefore, that the judge issuing the warrant had a “substantial basis for . . . conclud[ing]” that probable cause to search the Gateses’ home and car existed. The judgment of the Supreme Court of Illinois therefore must be
  Reversed.
  The apparent rule of Crowell v. Randell that a federal claim have been both raised and addressed in state court was generally not understood in the literal fashion in which it was phrased. See R. Robertson & F. Kirkham, Jurisdiction of the Supreme Court of the United States § 60 (1951). Instead, the Court developed the rule that a claim would not be considered here unless it had been either raised or squarely considered and resolved in state court. See, e. g., McGoldrick v. Compagnie Generale Transatlantique, 309 U. S. 430, 434-435 (1940); State Farm Mutual Ins. Co. v. Duel, 324 U. S. 154, 160 (1945).
  In Dewey, certain assessments had been levied against the owner of property abutting a street paved by the city; a state trial court ordered that the property be forfeited when the assessments were not paid, and in addition, held the plaintiff in error personally liable for the amount by which the assessments exceeded the value of the lots. In state court the plaintiff in error argued that the imposition of personal liability against him violated the Due Process Clause of the Fourteenth Amendment, because he had not received personal notice of the assessment proceedings. In this Court, he also attempted to argue that the assessment itself constituted a taking under the Fourteenth Amendment. The Court held that, beyond arising from a single factual occurrence, the two claims “are not in anywise necessarily connected,” 173 U. S., at 198. Because of this, we concluded that the plaintiff in error’s taking claim could not be considered.
  In Spinelli, police officers observed Mr. Spinelli going to and from a particular apartment, which the telephone company said contained two telephones with stated numbers. The officers also were “informed by a confidential reliable informant that William Spinelli [was engaging in illegal gambling activities]” at the apartment, and that he used two phones, with numbers corresponding to those possessed by the police. 393 U. S., at 414. The officers submitted an affidavit with this information to a magistrate and obtained a warrant to search Spinelli’s apartment. We held that the magistrate could have made his determination of probable cause only by “abdicating his constitutional function,” id., at 416. The Government’s affidavit contained absolutely no information regarding the informant’s reliability. Thus, it did not satisfy Aguilar*s requirement that such affidavits contain “some of the underlying circumstances” indicating that “the informant . . . was ‘credible’” or that “his information [was] ‘reliable.’” Aguilar v. Texas, 378 U. S. 108, 114 (1964). In addition, the tip failed to satisfy Aguilar’s requirement that it detail “some of the underlying circumstances from which the informant concluded that. . . narcotics were where he claimed they were.” Ibid. We also held that if the tip concerning Spinelli had contained “sufficient detail” to permit the magistrate to conclude “that he [was] relying on something more substantial than a casual rumor circulating in the underworld or an accusation based merely on an individual’s general reputation,” 393 U. S., at 416, then he properly could have relied on it; we thought, however, that the tip lacked the requisite detail to permit this “self-verifying detail” analysis.
  See, e. g., Stanley v. State, 19 Md. App. 507, 313 A. 2d 847 (1974). In summary, these rules posit that the “veracity” prong of the Spinelli test has two “spurs” — the informant’s “credibility” and the “reliability” of his information. Various interpretations are advanced for the meaning of the “reliability” spur of the “veracity” prong. Both the “basis of knowledge” prong and the “veracity” prong are treated as entirely separate requirements, which must be independently satisfied in every case in order to sustain a determination of probable cause. See n. 5, infra. Some ancillary doctrines are relied on to satisfy certain of the foregoing requirements. For example, the “self-verifying detail” of a tip may satisfy the “basis of knowledge” requirement, although not the “credibility” spur of the “veracity” prong. See 85 Ill. 2d, at 388, 423 N. E. 2d, at 892. Conversely, corroboration would seem not capable of supporting the “basis of knowledge” prong, but only the “veracity” prong. Id., at 390, 423 N. E. 2d, at 893.
  The decision in Stanley, while expressly approving and conscientiously attempting to apply the “two-pronged test” observes that “[t]he built-in subtleties [of the test] are such, however, that a slipshod application calls down upon us the fury of Murphy’s Law.” 19 Md. App., at 528, 313 A. 2d, at 860 (footnote omitted). The decision also suggested that it is necessary to “evolve analogous guidelines [to hearsay rules employed in trial settings] for the reception of hearsay in a probable cause setting.” Id., at 522, n. 12, 313 A. 2d, at 857, n. 12.
  The entirely independent character that the Spinelli prongs have assumed is indicated both by the opinion of the Illinois Supreme Court in this case, and by decisions of other courts. One frequently cited decision, Stanley v. State, supra, at 530, 313 A. 2d, at 861 (footnote omitted), remarks that “the dual requirements represented by the ‘two-pronged test’ are ‘analytically severable’ and an ‘overkill’ on one prong will not carry over to make up for a deficit on the other prong.” See also n. 9, infra.
  Our original phrasing of the so-called “two-pronged test” in Aguilar v. Texas, supra, suggests that the two prongs were intended simply as guides to a magistrate’s determination of probable cause, not as inflexible, independent requirements applicable in every case. In Aguilar, we required only that
  “the magistrate must be informed of some of the underlying circumstances from which the informant concluded that . . . narcotics were where he claimed they were, and some of the underlying circumstances from which the officer concluded that the informant. . . was ‘credible’ or his information ‘reliable.’” Id., at 114 (emphasis added).
  As our language indicates, we intended neither a rigid compartmentalization of the inquiries into an informant’s “veracity,” “reliability,” and “basis of knowledge,” nor that these inquiries be elaborate exegeses of an informant’s tip. Rather, we required only that some facts bearing on two particular issues be provided to the magistrate. Our decision in Jaben v. United States, 381 U. S. 214 (1965), demonstrated this latter point. We held there that a criminal complaint showed probable cause to believe the defendant had attempted to evade the payment of income taxes. We commented:
  “Obviously any reliance upon factual allegations necessarily entails some degree of reliability upon the credibility of the source.... Nor does it indicate that each factual allegation which the affiant puts forth must be independently documented, or that each and every fact which contributed to his conclusions be spelled out in the complaint. . . . It simply requires that enough information be presented to the Commissioner to enable him to make the judgment that the charges are not capricious and are sufficiently supported to justify bringing into play the further steps of the criminal process.” Id., at 224-225 (emphasis added).
  The diversity of informants’ tips, as well as the usefulness of the totality-of-the-circumstances approach to probable cause, is reflected in our prior decisions on the subject. In Jones v. United States, 362 U. S. 257, 271 (1960), we held that probable cause to search petitioners’ apartment was established by an affidavit based principally on an informant’s tip. The unnamed informant claimed to have purchased narcotics from petitioners at their apartment; the affiant stated that he had been given correct information from the informant on a prior occasion. This, and the fact that petitioners had admitted to police officers on another occasion that they were narcotics users, sufficed to support the magistrate’s determination of probable cause.
  Likewise, in Rugendorf v. United States, 376 U. S. 528 (1964), the Court upheld a magistrate’s determination that there was probable cause to believe that certain stolen property would be found in petitioner’s apartment. The affidavit submitted to the magistrate stated that certain furs had been stolen, and that a confidential informant, who previously had furnished confidential information, said that he saw the furs in petitioner’s home. Moreover, another confidential informant, also claimed to be reliable, stated that one Schweihs had stolen the furs. Police reports indicated that petitioner had been seen in Schweihs’ company, and a third informant stated that petitioner was a fence for Schweihs.
  Finally, in Ker v. California, 374 U. S. 23 (1963), we held that information within the knowledge of officers who searched the Kers’ apartment provided them with probable cause to believe drugs would be found there. The officers were aware that one Murphy had previously sold marihuana to a police officer; the transaction had occurred in an isolated area, to which Murphy had led the police. The night after this transaction, police observed Mr. Ker and Murphy meet in the same location. Murphy approached Ker’s car, and, although police could see nothing change hands, Murphy’s modus operandi was identical to what it had been the night before. Moreover, when police followed Ker from the scene of the meeting with Murphy he managed to lose them after performing an abrupt U-turn. Finally, the police had a statement from an informant who had provided reliable information previously, that Ker was engaged in selling marihuana, and that his source was Murphy. We concluded that “[t]o say that this coincidence of information was sufficient to support a reasonable belief of the officers that Ker was illegally in possession of marijuana is to indulge in understatement.” Id., at 36.
  Compare Stanley v. State, 19 Md. App., at 530, 313 A. 2d, at 861, reasoning that “[e]ven assuming ‘credibility’ amounting to sainthood, the judge still may not accept the bare conclusion ... of a sworn and known and trusted police-affiant.”
  Some lower court decisions, brought to our attention by the State, reflect a rigid application of such rules. In Bridger v. State, 503 S. W. 2d 801 (Tex. Crim. App. 1974), .the affiant had received a confession of armed robbery from one of two suspects in the robbery; in addition, the suspect had given the officer $800 in cash stolen during the robbery. The suspect also told the officer that the gun used in the robbery was hidden in the other suspect’s apartment. A warrant issued on the basis of this was invalidated on the ground that the affidavit did not satisfactorily describe how the accomplice had obtained his information regarding the gun.
  Likewise, in People v. Palanza, 55 Ill. App. 3d 1028, 371 N. E. 2d 687 (1978), the affidavit submitted in support of an application for a search warrant stated that an informant of proven and uncontested reliability had seen, in specifically described premises, “a quantity of a white crystalline substance which was represented to the informant by a white male occupant of the premises to be cocaine. Informant has observed cocaine on numerous occasions in the past and is thoroughly familiar with its appearance. The informant states that the white crystalline powder he observed in the above described premises appeared to him to be cocaine.” Id., at 1029, 371N. E. 2d, at 688. The warrant issued on the basis of the affidavit was invalidated because “[t]here is no indication as to how the informant or for that matter any other person could tell whether a white substance was cocaine and not some other substance such as sugar or salt.” Id., at 1030, 371 N. E. 2d, at 689.
  Finally, in People v. Brethauer, 174 Colo. 29, 482 P. 2d 369 (1971), an informant, stated to have supplied reliable information in the past, claimed that L. S. D. and marihuana were located on certain premises. The informant supplied police with drugs, which were tested by police and confirmed to be illegal substances. The affidavit setting forth these, and other, facts was found defective under both prongs of Spinelli.
  We also have said that “[a]lthough in a particular case it may not be easy to determine when an affidavit demonstrates the existence of probable cause, the resolution of doubtful or marginal cases in this area should be largely determined by the preference to be accorded to warrants,” United States v. Ventresca, 380 U. S. 102, 109 (1965). This reflects both a desire to encourage use of the warrant process by police officers and a recognition that once a warrant has been obtained, intrusion upon interests protected by the Fourth Amendment is less severe than otherwise may be the case. Even if we were to accept the premise that the accurate assessment of probable cause would be furthered by the “two-pronged test,” which we do not, these Fourth Amendment policies would require a less rigorous standard than that which appears to have been read into Aguilar and Spinelli.
  The Court’s decision in Spinelli has been the subject of considerable criticism, both by Members of this Court and others. Justice Blackmun, concurring in United States v. Harris, 403 U. S. 573, 585-586 (1971), noted his long-held view “that Spinelli . . . was wrongly decided” by this Court. Justice Black similarly would have overruled that decision. Id., at 585. Likewise, a noted commentator has observed that “[t]he Aguilar-Spinelli formulation has provoked apparently ceaseless litigation.” 8A J. Moore, Moore’s Federal Practice ¶ 41.04, p. 41-43 (1982).
  Whether the allegations submitted to the magistrate in Spinelli would, under the view we now take, have supported a finding of probable cause, we think it would not be profitable to decide. There are so many variables in the probable-cause equation that one determination will seldom be a useful “precedent” for another. Suffice it to say that while we in no way abandon Spinelli’s concern for the trustworthiness of informers and for the principle that it is the magistrate who must ultimately make a finding of probable cause, we reject the rigid categorization suggested by some of its language.
  The tip in Draper might well not have survived the rigid application of the “two-pronged test” that developed following Spinelli. The only reference to Hereford’s reliability was that he had “been engaged as a ‘special employee’ of the Bureau of Narcotics at Denver for about six months, and from time to time gave information to [the police for] small sums of money, and that [the officer] had always found the information given by Hereford to be accurate and reliable.” 358 U. S., at 309. Likewise, the tip gave no indication of how Hereford came by his information. At most, the detailed and accurate predictions in the tip indicated that, however Hereford obtained his information, it was reliable.
  The Illinois Supreme Court thought that the verification of details contained in the anonymous letter in this case amounted only to “[t]he corroboration of innocent activity,” 85 Ill. 2d 376, 390, 423 N. E. 2d 887, 893 (1981), and that this was insufficient to support a finding of probable cause. We are inclined to agree, however, with the observation of Justice Moran in his dissenting opinion that “[i]n this case, just as in Draper, seemingly innocent activity became suspicious in light of the initial tip.” Id.., at 396, 423 N. E. 2d, at 896. And it bears noting that all of the corroborating detail established in Draper was of entirely innocent activity — a fact later pointed out by the Court in both Jones v. United States, 362 U. S., at 269-270, and Ker v. California, 374 U. S., at 36.
  This is perfectly reasonable. As discussed previously, probable cause requires only a probability or substantial chance of criminal activity, not an actual showing of such activity. By hypothesis, therefore, innocent behavior frequently will provide the basis for a showing of probable cause; to require otherwise would be to sub silentio impose a drastically more rigorous definition of probable cause than the security of our citizens’ demands. We think the Illinois court attempted a too rigid classification of the types of conduct that may be relied upon in seeking to demonstrate probable cause. See Brown v. Texas, 443 U. S. 47, 52, n. 2 (1979). In making a determination of probable cause the relevant inquiry is not whether particular conduct is “innocent” or “guilty,” but the degree of suspicion that attaches to particular types of noncriminal acts.
  Justice Stevens’ dissent seizes on one inaccuracy in the anonymous informant’s letter — its statement that Sue Gates would fly from Florida to Illinois, when in fact she drove — and argues that the probative value of the entire tip was undermined by this allegedly “material mistake.” We have never required that informants used by the police be infallible, and can see no reason to impose such a requirement in this case. Probable cause, particularly when police have obtained a warrant, simply does not require the perfection the dissent finds necessary.
  Likewise, there is no force to the dissent’s argument that the Gateses’ action in leaving their home unguarded undercut the informant’s claim that drugs were hidden there. Indeed, the line-by-line scrutiny that the dissent applies to the anonymous letter is akin to that which we find inappropriate in reviewing magistrates’ decisions. The dissent apparently attributes to the judge who issued the warrant in this case the rather implausible notion that persons dealing in drugs always stay at home, apparently out of fear that to leave might risk intrusion by criminals. If accurate, one could not help sympathizing with the self-imposed isolation of people so situated. In reality, however, it is scarcely likely that the judge ever thought that the anonymous tip “kept one spouse” at home, much less that he relied on the theory advanced by the dissent. The letter simply says that Sue would fly from Florida to Illinois, without indicating whether the Gateses made the bitter choice of leaving the drugs in their house, or those in their car, unguarded. The judge’s determination that there might be drugs or evidence of criminal activity in the Gateses’ home was well supported by the less speculative theory, noted in text, that if the informant could predict with considerable accuracy the somewhat unusual travel plans of the Gateses, he probably also had a reliable basis for his statements that the Gateses kept a large quantity of drugs in their home and frequently were visited by other drug traffickers there.
choices: stay, petition, or motion granted affirmed reversed reversed and remanded vacated and remanded affirmed and reversed (or vacated) in part affirmed and reversed (or vacated) in part and remanded vacated petition denied or appeal dismissed modify remand unusual disposition