Merger and acquisition activity is extraordinarily prevalent in the United States and generates massive expenditures every year, but at its heart is a serious empirical puzzle. Despite many years of search, a range of empirical researchers have failed to find evidence that this activity generates benefits, either for the combining firms’ owners or for anyone else. While the problem is not unknown among academics, and the result is no longer very seriously contested, no one has any conclusive explanation for it. The law, for its part, ignores it entirely. This paper comprehensively reviews the empirical literature and the several confusing puzzles it raises, and works through the policy implications for the bodies of law it affects. The most important insights are two. First, the evidence casts doubt on the general confidence within corporations scholarship in the promise of oversight by robust market institutions, as an alternative to law and courts, especially the disciplinary power of the market for corporate control. Second, the less for antitrust law is simple. The fear of jeopardizing gains from merger, on which the antitrust law of mergers and acquisitions has been rendered largely inert, is no longer very defensible. The balance between false positive and false negative should be re-set.