Legal scholars and political scientists typically study domestic public law and international public law as separate subjects, treating them as essentially different phenomena. This separate treatment has been justified on the grounds that the two realms of public law have important and intrinsic structural and functional differences: domestic law is a hierarchical system with centralized enforcement whose primary function is to constrain behavior, whereas international law is an anarchic system relying on decentralized enforcement or self-help that primarily performs functions other than constraint. This article reassesses that understanding. It argues that domestic public law and international public law are in fact fundamentally similar in terms of doctrine and structure and, from the perspective of positive political theory, the two realms of public law function in similar ways. The article therefore proposes a unified concept of public law as an alternative to the traditional structural/functional distinction, and demonstrates the theoretical, pedagogical and policy benefits of using the unified concept to think across the domestic-international divide in the study of public law.