Custom is an underdeveloped concept in Thirteenth Amendment jurisprudence. While a substantial body of work has explored the technical meaning of custom as it applies to § 1983 and, to a lesser extent, Congress’s power to enforce the Fourteenth Amendment, few scholars have offered sustained treatment of custom as a way to understand the meaning and scope of the Thirteenth Amendment. This gap exists despite the fact that Congress specifically identified custom as a subject of regulation when it passed the Civil Rights Act of 1866 and despite the fact that the Thirteenth Amendment operates directly on the behavior of private parties. The fact that the Thirteenth Amendment can be applied to custom has important implications for how the Amendment should be construed. In particular, the concept of custom — especially as it relates to practices that upheld the slave system in the South — helps give shape and content to the other undefined terms the Thirteenth Amendment has generated: the “badges,” “incidents,” and “relics” of slavery. Ultimately, the concept of custom can help guide policymakers and judges who must consider the scope, the limitations, and the continuing relevance of the Thirteenth Amendment in the twenty-first century.