Can a company block a specific person from buying its stock?
I assume you are talking about a publicly traded company listed on a major stock exchange and the buyer resides in the US. (Private companies and non-US locations can change the rules really a lot.) The short answer is no, because the company does not own the stock, various investors do. Each investor has to make an individual decision to sell or not sell. But there are complications. If an entity buys more than about 10% of the company they have to file a declaration with the SEC. The limit can be higher if they file an assertion that they are buying it solely for investment and are not seeking control of the company. If they are seeking control of the company then more paperwork must be filed and if they want to buy the whole company they may be required to make a tender offer where they offer to buy any and all shares at a specific price. If the company being bought is a financial institution, then the buyer may have to declare as a bank holding company and more regulations apply. The company can advise shareholders not to take the tender offer, but they cannot forbid it. So the short answer is, below 10% and for investment purposes only, it is cash and carry: Whoever has the cash gets to carry the stock away. Above that various regulations and declarations apply, but the company still does not have the power prevent the purchase in most circumstances. "A more serious problem: how do you know who's really buying your stock? ""Shell companies"" are an increasingly obvious problem in corporate and tax accountability. There are jurisdictions where companies can be created with secret lists of directors and shareholders. If stock is bought by one of these companies, it is very hard to trace it to a particular individual." In the UK, this is the very definition of a Public Limited Company.    A Limited Company can restrict how its stock is trades and who can buy and sell and when, a Public Limited Company cannot. Most stock exchanges will only allow Public Limited Company stock to be traded. Therefore a company can control who its stock holders are or be traded on a Stock Exchange. The company could use registered shares with restricted transferability, i.e. shares that require the consent of the issuing company for a change of ownership. The answer to this question is given by the fact that many public companies have people who are opposed to the company's aims or practices and who own their stock, often a single share, for the purposes of turning up to shareholder meetings and haranguing directors/asking awkward questions/disrupting proceedings, etc. If public companies could stop these campaigning shareholders from owning stock they would.
"Something to note is that when a company announces a share buyback program there is usually a time frame and amount of shares that are important details as it isn't like the company will make one big buy back of stock generally.  Rather it may take months or even years as noted in the Wikipedia article about share repurchases. Wikipedia covers some of the technical details here but to give a specific set of answers: When a company announces a share buyback program, who do they actually   buy back the shares from?  From the Wikipedia link: ""Under US corporate law there are five primary methods of stock repurchase: open market, private negotiations, repurchase 'put' rights, and two variants of self-tender repurchase: a fixed price tender offer and a Dutch auction."" Thus, there are open market and a couple of other possibilities. Openly traded shares on a stock exchange?  Possibly, though there are other options. Is there a fixed price that   they buy back at? Sometimes.  I'd think a ""fixed price tender offer"" would imply a fixed price though the open market way may take various prices. If I own shares in that company, can I get them to buy back my shares? Selective Buy-Backs is noted in Wikipedia as: ""In broad terms, a selective buy-back is one in which identical offers are not made to every shareholder, for example, if offers are made to only some of the shareholders in the company. In the US, no special shareholder approval of a selective buy-back is required. In the UK, the scheme must first be approved by all shareholders, or by a special resolution (requiring a 75% majority) of the members in which no vote is cast by selling shareholders or their associates. Selling shareholders may not vote in favor of a special resolution to approve a selective buy-back. The notice to shareholders convening the meeting to vote on a selective buy-back must include a statement setting out all material information that is relevant to the proposal, although it is not necessary for the company to provide information already disclosed to the shareholders, if that would be unreasonable."" Thus it is possible, though how probable is another question. While not in the question, something to consider is how the buybacks can be done as a result of offsetting the dilution of employees who have stock options that may exercise them and spread the earnings over more shares, but this is more on understanding the employee stock option scenario that various big companies use when it comes to giving employees an incentive to help the stock price." A stock buy back reduces the number of stocks available on the open market.  Since stocks are literally a share in ownership a buy back of the stock then when the company repurchases it has the effect of increasing the percent of ownership of the company of each stock. Zynga has a Market Cap of ~1810M so a 200M buy back will increase the ownership value of each stock by ~12%.  This has had the effect of an immediate stock price bump of around 12% which is to be expected as the value becomes the expected post buyback value. However long term gains will require Zynga to turn around their business.  This bump will only be sustainable if they can.  If their business continues to decline then its stock price will continue to slide.  There are some who would rather see Zynga invest that 200m in getting a new product to market to bring revenues up rather than spending precious capital on a plan to temporarily bump a stock that is headed towards the floor.  If on the other hand the revenue is poised to recover and the company has the excess capitol buying back stock low is a great way to get the most back for your shareholders bucks. Can they repurchase at any price and any time? They can write a buy order for any price at any time in the future, though they have some restrictions from the SEC mostly involving disclosures.  But it is up to the sellers to choose to sell at that price.  If they execute the buy back at a rate comparable to market rate then they are more likely to get takers than if they attempt to buy it back at a significant reduction from market price.  So since today(10-25-2012) the it is selling for ~2.30  A buy order for 2.30 is going to get more action than one at 2.00.  Investors will often look at the companies buy back offer for a company in decline(like Zynga has been) as the true value of the company.  If so then a lowball buyback offer could add downward pressure on the stock price. You can execute block trades on the options market and get exercised for shares to create a very large position in Energy Transfer Partners LP without moving the stock market. You can then place limit sell orders, after selling directly into the market and keep an overhang of low priced shares (the technical analysis traders won't know what you specifically are doing, and will call this 'resistance'). If you hit nice even numbers (multiples of 5, multiples of 10) with your sell orders, you can exacerbate selling as many market participants will have their own stop loss orders at those numbers, causing other people to sell at lower and lower prices automatically, and simultaneously keep your massive ask in effect. If your position is bigger than the demand then you can keep a stock lower. The secondary market doesn't inherently affect a company in any way. But many companies have borrowed against the price of their shares, and if you get the share price low enough they can get suddenly margin called and be unable to service their existing debt. You will also lose a lot of money doing this, so you can also buy puts along the way or attempt to execute a collar to lower your own losses. The collar strategy is nice because it is unlikely that other traders and analysts will notice what you are doing, since there are calls, puts and share orders involved in creating it. One person may notice the block trade for the calls initially, but nobody will notice it is part of a larger strategy with multiple legs. With the share position, you may also be able to vote on some things, but that solely depends on the conditions of the shares. "They do but you're missing some calculations needed to gain an understanding. Intro To Stock Index Weighting Methods notes in part: Market cap is the most common weighting method used by an index.   Market cap or market capitalization is the standard way to measure the   size of the company. You might have heard of large, mid, or small cap   stocks? Large cap stocks carry a higher weighting in this index. And   most of the major indices, like the S&P 500, use the market cap   weighting method. Stocks are weighted by the proportion of their market cap to the total   market cap of all the stocks in the index. As a stock’s price and   market cap rises, it gains a bigger weighting in the index. In turn   the opposite, lower stock price and market cap, pushes its weighting   down in the index.  Pros Proponents argue that large companies have a bigger effect on the   economy and are more widely owned. So they should have a bigger   representation when measuring the performance of the market. Which is   true. Cons It doesn’t make sense as an investment strategy. According to a market   cap weighted index, investors would buy more of a stock as its price   rises and sell the stock as the price falls. This is the exact   opposite of the buy low, sell high mentality investors should use. Eventually, you would have more money in overpriced stocks and less in   underpriced stocks. Yet most index funds follow this weighting method. Thus, there was likely a point in time where the S & P 500's initial sum was equated to a specific value though this is the part you may be missing here. Also, how do you handle when constituents change over time? For example, suppose in the S & P 500 that a $100,000,000 company is taken out and replaced with a $10,000,000,000 company that shouldn't suddenly make the index jump by a bunch of points because the underlying security was swapped or would you be cool with there being jumps when companies change or shares outstanding are rebalanced? Consider carefully how you answer that question. In terms of histories, Dow Jones Industrial Average and S & P 500 Index would be covered on Wikipedia where from the latter link: The ""Composite Index"",[13] as the S&P 500 was first called when it   introduced its first stock index in 1923, began tracking a small   number of stocks. Three years later in 1926, the Composite Index   expanded to 90 stocks and then in 1957 it expanded to its current   500.[13] Standard & Poor's, a company that doles out financial information and analysis, was founded in 1860 by Henry Varnum Poor. In   1941 Poor's Publishing (Henry Varnum Poor's original company) merged   with Standard Statistics (founded in 1906 as the Standard Statistics   Bureau) and therein assumed the name Standard and Poor's Corporation.   The S&P 500 index in its present form began on March 4, 1957.   Technology has allowed the index to be calculated and disseminated in   real time. The S&P 500 is widely used as a measure of the general   level of stock prices, as it includes both growth stocks and value   stocks. In September 1962, Ultronic Systems Corp. entered into an agreement   with Standard and Poor's. Under the terms of this agreement, Ultronics   computed the S&P 500 Stock Composite Index, the 425 Stock Industrial   Index, the 50 Stock Utility Index, and the 25 Stock Rail Index.   Throughout the market day these statistics were furnished to Standard   & Poor's. In addition, Ultronics also computed and reported the 94 S&P   sub-indexes.[14] There are also articles like Business Insider that have this graphic that may be interesting: S & P changes over the years The makeup of the S&P 500 is constantly changing notes in part: ""In most years 25 to 30 stocks in the S&P 500 are replaced,"" said   David Blitzer, S&P's Chairman of the Index Committee. And while there   are strict guidelines for what companies are added, the final decision   and timing of that decision depends on what's going through the heads   of a handful of people employed by Dow Jones." RonJohn is right, all shares are owned by someone.  Depending on the company, they can be closely held so that nobody wants to sell at a given time.  This can cause the price people are offering to rise until someone sells.  That trade will cause an adjustment in the ticker price of that stock.  Supply and demand at work.  Berkshire Hathaway is an example of this.  The number of shares is low, the demand for them is high, the price per share is high. sweet, a Nazi analogy.  And here I thought I'd *never* see one of those on the internet!  A better example would be to arrest gun manufacturers because they created guns that were used in murders.  Chances are this program was created for legitimate reasons. There's nothing illegal about using software to buy stock. There is, however, rules against doing things like this to influence the market.   You're equating some software developers whose software was used illegally to a group of people who committed genocide.  Think about that. If this is a publicly traded company, I'd be thinking the shareholders should take a long hard look at this.  This is a man who hates his employees more than he likes money.  A spite-based decision is obviously going to be inferior to a money-based decision.  And shareholders want money. "This is an old question, but a new product has popped up that provides an alternative answer. There is a website called stockpile.com that allows you to purchase ""stock gift certificates"" for others.  These come in both electronic and traditional physical form.  This meets my question's original criteria of a gift giver paying for stock without having any of the recipient's personal information and thus maintaining the gift's surprise. I should note a few things about this service: Despite these limitations I wanted to post it here so others were aware of it as an option.  If no other alternative will work and this is what it takes to get a parent interested in teaching their child to invest, then it's well worth the costs."