If corporate governance matters for firm performance and this relationship is fully incorporated by the market, then a stock price should quickly adjust to any relevant change in the firm's governance. This is the logic behind the use of event studies to analyze the impact of takeover defenses. If such a reaction occurs, then expected returns on the stock would be unaffected beyond the event window. However, if governance matters but is not incorporated immediately into stock prices, then realized returns on the stock would differ systematically from equivalent securities.