Justice Scalia
delivered the opinion of the Court.
Petitioner Joseph Griffin, who was on probation, had his home searched by probation officers acting without a warrant. The officers found a gun that later served as the basis of Griffin’s conviction of a state-law weapons offense. We granted certiorari, 479 U. S. 1005 (1986), to consider whether this search violated the Fourth Amendment.
I
On September 4, 1980, Griffin, who had previously been convicted of a felony, was convicted in Wisconsin state court of resisting arrest, disorderly conduct, and obstructing an officer. He was placed on probation.
Wisconsin law puts probationers in the legal custody of the State Department of Health and Social Services and renders them “subject . . . to . . . conditions set by the court and rules and regulations established by the department.” Wis. Stat. § 973.10(1) (1985-1986). One of the Department’s regulations permits any probation officer to search a probationer’s home without a warrant as long as his supervisor approves and as long as there are “reasonable grounds” to believe the presence of contraband — including any item that the probationer cannot possess under the probation conditions. Wis. Admin. Code HSS §§ 328.21(4), 328.16(1) (1981). The rule provides that an officer should consider a variety of factors in determining whether “reasonable grounds” exist, among which are information provided by an informant, the reliability and specificity of that information, the reliability of the informant (including whether the informant has any incentive to supply inaccurate information), the officer’s own experience with the probationer, and the “need to verify compliance with rules of supervision and state and federal law.” HSS §328.21(7). Another regulation makes it a violation of the terms of probation to refuse to consent to a home search. HSS § 328.04(3)(k). And still another forbids a probationer to possess a firearm without advance approval from a probation officer. HSS § 328.04(3)(j).
On April 5, 1983, while Griffin was still on probation, Michael Lew, the supervisor of Griffin’s probation officer, received information from a detective on the Beloit Police Department that there were or might be guns in Griffin’s apartment. Unable to secure the assistance of Griffin’s own probation officer, Lew, accompanied by another probation officer and three plainclothes policemen, went to the apartment. When Griffin answered the door, Lew told him who they were and informed him that they were going to search his home. During the subsequent search — carried out entirely by the probation officers under the authority of Wisconsin’s probation regulation — they found a handgun.
Griffin was charged with possession of a firearm by a convicted felon, which is itself a felony. Wis. Stat. §941.29(2) (1985-1986). He moved to suppress the evidence seized during the search. The trial court denied the motion, concluding that no warrant was necessary and that the search was reasonable. A jury convicted Griffin of the firearms violation, and he was sentenced to two years’ imprisonment. The conviction was affirmed by the Wisconsin Court of Appeals, 126 Wis. 2d 183, 376 N. W. 2d 62 (1985).
On further appeal, the Wisconsin Supreme Court also affirmed. It found denial of the suppression motion proper because probation diminishes a probationer’s reasonable expectation of privacy — so that a probation officer may, consistent with the Fourth Amendment, search a probationer’s home without a warrant, and with only “reasonable grounds” (not probable cause) to believe that contraband is present. It held that the “reasonable grounds” standard of Wisconsin’s search regulation satisfied this “reasonable grounds” standard of the Federal Constitution, and that the detective’s tip established “reasonable grounds” within the meaning of the regulation, since it came from someone who had no reason to supply inaccurate information, specifically identified Griffin, and suggested a need to verify Griffin’s compliance with state law. 131 Wis. 2d 41, 52-64, 388 N. W. 2d 535, 539-544 (1986).
II
We think the Wisconsin Supreme Court correctly concluded that this warrantless search did not violate the Fourth Amendment. To reach that result, however, we find it unnecessary to embrace a new principle of law, as the Wisconsin court evidently did, that any search of a probationer’s home by a probation officer satisfies the Fourth Amendment as long as the information possessed by the officer satisfies a federal “reasonable grounds” standard. As his sentence for the commission of a crime, Griffin was committed to the legal custody of the Wisconsin State Department of Health and Social Services, and thereby made subject to that Department’s rules and regulations. The search of Griffin’s home satisfied the demands of the Fourth Amendment because it was carried out pursuant to a regulation that itself satisfies the Fourth Amendment’s reasonableness requirement under well-established principles.
A
A probationer’s home, like anyone else’s, is protected by the Fourth Amendment’s requirement that searches be “reasonable.” Although we usually require that a search be undertaken only pursuant to a warrant (and thus supported by probable cause, as the Constitution says warrants must be), see, e. g., Payton v. New York, 445 U. S. 573, 586 (1980), we have permitted exceptions when “special needs, beyond the normal need for law enforcement, make the warrant and probable-cause requirement impracticable.” New Jersey v. T. L. O., 469 U. S. 325, 351 (1985) (Blackmun, J., concurring in judgment). Thus, we have held that government employers and supervisors may conduct warrantless, work-related searches of employees’ desks and offices without probable cause, O’Connor v. Ortega, 480 U. S. 709 (1987), and that school officials may conduct warrantless searches of some student property, also without probable cause, New Jersey v. T. L. O., swpra. We have also held, for similar reasons, that in certain circumstances government investigators conducting searches pursuant to a regulatory scheme need not adhere to the usual warrant or probable-cause requirements as long as their searches meet “reasonable legislative or administrative standards.” Camara v. Municipal Court, 387 U. S. 523, 538 (1967). See New York v. Burger, 482 U. S. 691, 702-703 (1987); Donovan v. Dewey, 452 U. S. 594, 602 (1981); United States v. Biswell, 406 XJ. S. 311, 316 (1972).
A State’s operation of a probation system, like its operation of a school, government office or prison, or its supervision of a regulated industry, likewise presents “special needs” beyond normal law enforcement that may justify departures from the usual warrant and probable-cause requirements. Probation, like incarceration, is “a form of criminal sanction imposed by a court upon an offender after verdict, finding, or plea of guilty.” G. Killinger, H. Kerper, & P. Cromwell, Probation and Parole in the Criminal Justice System 14 (1976); see also 18 U. S. C. § 3651 (1982 ed. and Supp. III) (probation imposed instead of imprisonment); Wis. Stat. § 973.09 (1985-1986) (same). Probation is simply one point (or, more accurately, one set of points) on a continuum of possible punishments ranging from solitary confinement in a maximum-security facility to a few hours of mandatory community service. A number of different options lie between those extremes, including confinement in a medium- or minimum-security facility, work-release programs, “halfway houses,” and probation — which can itself be more or less confining depending upon the number and severity of restrictions imposed. See, e. g., 18 U. S. C. §3563 (1982 ed., Supp. III) (effective Nov. 1, 1987) (probation conditions authorized in federal system include requiring probationers to avoid commission of other crimes; to pursue employment; to avoid certain occupations, places, and people; to spend evenings or weekends in prison; and to avoid narcotics or excessive use of alcohol). To a greater or lesser degree, it is always true of probationers (as we have said it to be true of parolees) that they do not enjoy “the absolute liberty to which every citizen is entitled, but only . . . conditional liberty properly dependent on observance of special [probation] restrictions.” Morrissey v. Brewer, 408 U. S. 471, 480 (1972).
These restrictions are meant to assure that the probation serves as a period of genuine rehabilitation and that the community is not harmed by the probationer’s being at large. See State v. Tarrell, 74 Wis. 2d 647, 652-653, 247 N. W. 2d 696, 700 (1976). These same goals require and justify the exercise of supervision to assure that the restrictions are in fact observed. Recent research suggests that more intensive supervision can reduce recidivism, see Petersilia, Probation and Felony Offenders, 49 Fed. Probation 9 (June 1985), and the importance of supervision has grown as probation has become an increasingly common sentence for those convicted of serious crimes, see id., at 4. Supervision, then, is a “special need” of the State permitting a degree of impingement upon privacy that would not be constitutional if applied to the public at large. That permissible degree is not unlimited, however, so we next turn to whether it has been exceeded here.
B
In determining whether the “special needs” of its probation system justify Wisconsin’s search regulation, we must take that regulation as it has been interpreted by state corrections officials and state courts. As already noted, the Wisconsin Supreme Court — the ultimate authority on issues of Wisconsin law — has held that a tip from a police detective that Griffin “had” or “may have had” an illegal weapon at his home constituted the requisite “reasonable grounds.” See 131 Wis. 2d, at 64, 388 N. W. 2d, at 544. Whether or not we would choose to interpret a similarly worded federal regulation in that fashion, we are bound by the state court’s interpretation, which is relevant to our constitutional analysis only insofar as it fixes the meaning of the regulation. We think it clear that the special needs of Wisconsin’s probation system make the warrant requirement impracticable and justify replacement of the standard of probable cause by “reasonable grounds,” as defined by the Wisconsin Supreme Court.
A warrant requirement would interfere to an appreciable degree with the probation system, setting up a magistrate rather than the probation officer as the judge of how close a supervision the probationer requires. Moreover, the delay inherent in obtaining a warrant would make it more difficult for probation officials to respond quickly to evidence of misconduct, see New Jersey v. T. L. O., 469 U. S., at 340, and would reduce the deterrent effect that the possibility of expeditious searches would otherwise create, see New York v. Burger, 482 U. S., at 710; United States v. Biswell, 406 U. S., at 316. By way of analogy, one might contemplate how parental custodial authority would be impaired by requiring judicial approval for search of a minor child’s room. And on the other side of the equation — the effect of dispensing with a warrant upon the probationer: Although a probation officer is not an impartial magistrate, neither is he the police officer who normally conducts searches against the ordinary citizen. He is an employee of the State Department of Health and Social Services who, while assuredly charged with protecting the public interest, is also supposed to have in mind the welfare of the probationer (who in the regulations is called a “client,” HSS § 328.03(5)). The applicable regulations require him, for example, to “[p]rovid[e] individualized counseling designed to foster growth and development of the client as necessary,” HSS § 328.04(2)(i), and “[m]onito[r] the client’s progress where services are provided by another agency and evaluate] the need for continuation of the services,” HSS §328.04(2)(o). In such a setting, we think it reasonable to dispense with the warrant requirement.
Justice Blackmun’s dissent would retain a judicial warrant requirement, though agreeing with our subsequent conclusion that reasonableness of the search does not require probable cause. This, however, is a combination that neither the text of the Constitution nor any of our prior decisions permits. While it is possible to say that Fourth Amendment reasonableness demands probable cause without a judicial warrant, the reverse runs up against the constitutional provision that “no Warrants shall issue, but upon probable cause.” Arndt. 4. The Constitution prescribes, in other words, that where the matter is of such a nature as to require a judicial warrant, it is also of such a nature as to require probable cause. Although we have arguably come to permit an exception to that prescription for administrative search warrants, which may but do not necessarily have to be issued by courts, we have never done so for constitutionally mandated judicial warrants. There it remains true that “[i]f a search warrant be constitutionally required, the requirement cannot be flexibly interpreted to dispense with the rigorous constitutional restrictions for its issue.” Frank v. Maryland, 359 U. S. 360, 373 (1959). Justice Blackmun neither gives a justification for departure from that principle nor considers its implications for the body of Fourth Amendment law.
We think that the probation regime would also be unduly disrupted by a requirement of probable cause. To take the facts of the present case, it is most unlikely that the unauthenticated tip of a police officer — bearing, as far as the record shows, no indication whether its basis was firsthand knowledge or, if not, whether the firsthand source was reliable, and merely stating that Griffin “had or might have” guns in his residence, not that he certainly had them — would meet the ordinary requirement of probable cause. But this is different from the ordinary case in two related respects: First, even more than the requirement of a warrant, a probable-cause requirement would reduce the deterrent effect of the supervisory arrangement. The probationer would be assured that so long as his illegal (and perhaps socially dangerous) activities were sufficiently concealed as to give rise to no more than reasonable suspicion, they would go undetected and uncorrected. The second difference is well reflected in the regulation specifying what is to be considered “[i]n deciding whether there are reasonable grounds to believe ... a client’s living quarters or property contain contraband,” HSS §328.21(7). The factors include not only the usual elements that a police officer or magistrate would consider, such as the detail and consistency of the information suggesting the presence of contraband and the reliability and motivation to dissemble of the informant, HSS §§328.21(7) (c), (d), but also “[ijnformation provided by the client which is relevant to whether the client possesses contraband,” and “[t]he experience of a staff member with that client or in a similar circumstance.” HSS §§ 328.21(7)(f), (g). As was true, then, in O’Connor v. Ortega, 480 U. S. 709 (1987), and New Jersey v. T. L. O., 469 U. S. 325 (1985), we deal with a situation in which there is an ongoing supervisory relationship —and one that is not, or at least not entirely, adversarial— between the object of the search and the decisionmaker.
In such circumstances it is both unrealistic and destructive of the whole object of the continuing probation relationship to insist upon the same degree of demonstrable reliability of particular items of supporting data, and upon the same degree of certainty of violation, as is required in other contexts. In some cases — especially those involving drugs or illegal weapons — the probation agency must be able to act based upon a lesser degree of certainty than the Fourth Amendment would otherwise require in order to intervene before a probationer does damage to himself or society. The agency, moreover, must be able to proceed on the basis of its entire experience with the probationer, and to assess probabilities in the light of its knowledge of his life, character, and circumstances.
To allow adequate play for such factors, we think it reasonable to permit information provided by a police officer, whether or not on the basis of firsthand knowledge, to support a probationer search. The same conclusion is suggested by the fact that the police máy be unwilling to disclose their confidential sources to probation personnel. For the same reason, and also because it is the very assumption of the institution of probation that the probationer is in need of rehabilitation and is more likely than the ordinary citizen to violate the law, we think it enough if the information provided indicates, as it did here, only the likelihood (“had or might have guns”) of facts justifying the search.
The search of Griffin’s residence was “reasonable” within the meaning of the Fourth Amendment because it was conducted pursuant to a valid regulation governing probationers. This conclusion makes it unnecessary to consider whether, as the court below held and the State urges, any search of a probationer’s home by a probation officer is lawful when there are “reasonable grounds” to believe contraband is present. For the foregoing reasons, the judgment of the Wisconsin Supreme Court is
Affirmed.
HSS § 328 was promulgated in December 1981 and became effective on January 1, 1982. Effective May 1, 1986, HSS § 328.21 was repealed and repromulgated with somewhat different numbering and without relevant substantive changes. See 131 Wis. 2d 41, 60, n. 7, 388 N. W. 2d 535, 542, n. 7 (1986). This opinion will cite the old version of § 328.21, which was in effect at the time of the search.
We have recently held that prison regulations allegedly infringing constitutional rights are themselves constitutional as long as they are “ ‘reasonably related to legitimate penological interests.’” O’Lone v. Estate of Shabazz, 482 U. S. 342, 349 (1987) (quoting Turner v. Safley, 482 U. S. 78, 89 (1987)). We have no occasion in this case to decide whether, as a general matter, that test applies to probation regulations as well.
If the regulation in question established a standard of conduct to which the probationer had to conform on pain of penalty — e. g., a restriction on his movements — the state court could not constitutionally adopt so unnatural an interpretation of the language that the regulation would fail to provide adequate notice. Cf. Kolender v. Lawson, 461 U. S. 352, 357-358 (1983); Lambert v. California, 355 U. S. 225, 228 (1957). That is not an issue here since, even though the petitioner would be in violation of his probation conditions (and subject to the penalties that entails) if he failed to consent to any search that the regulation authorized, see HSS §328.04(3)(k), nothing in the regulation or elsewhere required him to be advised, at the time of the request for search, what the probation officer’s “reasonable grounds” were, any more than the ordinary citizen has to be notified of the grounds for “probable cause” or “exigent circumstances” searches before they may be undertaken.
In the administrative search context, we formally require that administrative warrants be supported by “probable cause,” because in that context we use that term as referring not to a quantum of evidence, but merely to a requirement of reasonableness. See, e. g., Marshall v. Barlow’s, Inc., 436 U. S. 307, 320 (1978); Camara v. Municipal Court, 387 U. S. 523, 528 (1967). In other contexts, however, we use “probable cause” to refer to a quantum of evidence for the belief justifying the search, to be distinguished from a lesser quantum such as “reasonable suspicion.” See O’Connor v. Ortega, 480 U. S. 709, 724 (1987) (plurality); New Jersey v. T. L. O., 469 U. S. 325, 341-342 (1985). It is plainly in this sense that the dissent uses the term. See, e. g., post, at 881-883 (less than probable cause means “a reduced level of suspicion”).
5 See Marshall v. Barlow’s, Inc., supra, at 307 (“We hold that. . . the Act is unconstitutional insofar as it purports to authorize inspections without warrant or its equivalent”). The “neutral magistrate,” Camara, supra, at 532, or “neutral officer,” Marshall v. Barlow’s, Inc., supra, at 323, envisioned by our administrative search cases is not necessarily the “neutral judge,” post, at 887, envisioned by the dissent.
It is irrelevant whether the probation authorities relied upon any peculiar knowledge which they possessed of petitioner in deciding to conduct the present search. Our discussion pertains to the reasons generally supporting the proposition that the search decision should be left to the expertise of probation authorities rather than a magistrate, and should be supportable by a lesser quantum of concrete evidence justifying suspicion than would be required to establish probable cause. That those reasons may not obtain in a particular case is of no consequence. We may note, nonetheless, that the dissenters are in error to assert as a fact that the probation authorities made no use of special knowledge in the present case, post, at 890. All we know for certain is that the petitioner’s probation officer could not be reached; whether any material contained in petitioner’s probation file was used does not appear.
The dissenters speculate that the information might not have come from the police at all, “but from someone impersonating an officer.” Post, at 888. The trial court, however, found as a matter of fact that Lew received the tip on which he relied from a police officer. See 131 Wis. 2d, at 62, 388 N. W. 2d, at 543. The Wisconsin Supreme Court affirmed that finding, ibid., and neither the petitioner nor the dissenters assert that it is clearly erroneous.
The dissenters assert that the search did not comport with all the governing Wisconsin regulations. There are reasonable grounds on which the Wisconsin court could find that it did. But we need not belabor those here, since the only regulation upon which we rely for our constitutional decision is that which permits a warrantless search on “reasonable grounds.” The Wisconsin Supreme Court found the requirement of “reasonable grounds” to have been met on the facts of this case and, as discussed earlier, we hold that such a requirement, so interpreted, meets constitutional minimum standards as well. That the procedures followed, although establishing “reasonable grounds” under Wisconsin law, and adequate under federal constitutional standards, may have violated Wisconsin state regulations, is irrelevant to the ease before us.