Custom, including industry practices and social norms, has a tremendous influence on intellectual property (‘IP’) law, from affecting what happens outside of the courts in the trenches of the creative, technology, and science-based industries, to influencing how courts analyse infringement and defenses in IP cases. For decades, many scholars overlooked or dismissed the impact of custom on IP law in large part because of a belief that the dominant statutory frameworks that govern IP left little room for custom to play a role. In the last ten years, however, the landscape has shifted and more attention has been given to considering how custom affects IP entitlements both outside and inside the courtroom. This book chapter in the Research Handbook on the Economics of Intellectual Property Law focuses on the theoretical frames that inform the incorporation of custom into the law, and documents some of the practices and norms of various communities that use IP. I criticize the frequent and unreflected reliance on custom to determine the scope of IP rights, suggesting guidelines for when it can nevertheless be a sometimes useful tool for providing insights about IP laws. This analysis is partially informed by traditional common law limits on the incorporation of custom into the law. The chapter concludes with some recommendations for future areas of research for scholars based on my framework for thinking about custom in the context of IP laws.