We wrote this brief essay as an introduction to the volume “Customary Law” (Elgar, 2013). We briefly review several seminal works on customary law. Although scholars in law, economics, and history have provided a wealth of research on the subject, the issues arising from the intersection of customs and the law are far from settled. The discussion is divided into three parts. Part I examines a variety of perspectives on the history of customary law. Part II considers the role and limits of customary systems in a variety of commercial settings. Along the way it focuses on defining the proper relationship between law and custom from a law and economic perspective. Part III explores the role of custom in international law, from a broad set of analytical viewpoints.