This is a survey of the economic principles that underlie antitrust law and how those principles relate to competition policy. We address four core subject areas: market power, collusion, mergers between competitors, and monopolization. In each area, we select the most relevant portions of current economic knowledge and use that knowledge to critically assess central features of antitrust policy. Our objective is to foster the improvement of legal regimes and also to identify topics where further analytical and empirical exploration would be useful. This is a survey of the economic principles that underlie antitrust law and how those principles relate to competition policy. We address four core subject areas: market power, collusion, mergers between competitors, and monopolization. In each area, we select the most relevant portions of current economic knowledge and use that knowledge to critically assess central features of antitrust policy. Our objective is to foster the improvement of legal regimes and also to identify topics where further analytical and empirical exploration would be useful.