We quantitatively assess the role of activism in the market for corporate control by developing and estimating a model featuring both activism and M&A. We find that activism complements M&A, reducing the agency frictions associated with takeovers. However, activism simultaneously crowds out some M&A activity by substituting for disciplinary takeovers. Both the threat of activism and actual activist intervention create shareholder value by improving CEO incentives, while the value from reduced takeover frictions is primarily captured by acquirers. We find that activists have an information advantage, which is critical to overcoming the free rider problem in activist intervention.