Following an introduction of key terms and concepts, this chapter explains the significance of the interaction since the inception of competition law. The origins of US antitrust closely relate to corporate issues regarding trusts. While it marked its substantive reach outside the boundaries of the firm in Copperweld, it has an established tradition of cartel criminalisation against individuals. EU competition law, which emerged as part of the market integration project, is directed to undertakings, and enforced with general indifference to internal matters. Recent trends show the growing importance of internal incentives in cartel compliance. Competition law also faces the increasing complexity of corporate arrangements, exemplified by the delicate questions of liability in corporate groups and anticompetitive effects of minority shareholdings. This book thus opens the ‘black box’ of the firm, as conceived in competition law, with legal and economic inquiry into the realm of corporate governance. This chapter introduces the law and economics methodology of the book. Although centred on the EU, developments include consideration of the US and EU member states jurisdictions when relevant. Part I focuses on the substantive dimension of the interaction, Part II on enforcement aspects, with a focus on cartel practices.