Companies sometimes announce plans to buy back their own stock, but they don't always follow through. Managers usually do this when they think their stock is undervalued. A study looked at over 800 of these programs in the U.S. from 1980 to 1990. The stock market usually reacts positively to these announcements, especially when the repurchase is big and the stock's price is volatile. Interestingly, the market reacts less positively when the stock price doesn't match its true value.