A Practice Note discussing municipal liability in Section 1983 claims, under the US Supreme Court's decision in Monell v. Dep't of Soc. Servs., 436 U.S. 658 (1978). This Note covers theories of establishing a municipal policy or custom, such as formal policies, de facto policies and customs, failure to train and supervise, and single decisions made by final policymakers. This Note also covers how municipal policies or customs cause underlying constitutional violations and provides practice pointers when litigating Monell claims.