This Element advances an agency-theoretic approach to public administration through comparative analysis of the United States, China, and EU. It examines how principals – such as legislatures, executives, or ruling parties – can align the actions of diverse agents, including civil servants, public agencies, street-level bureaucrats, and contractors, with the public interest. Drawing on an extensive review of 146 key studies and AI-assisted analysis of 8,400 articles from Public Administration Review, Part I outlines fundamental concepts: goal divergence, moral hazard, adverse selection, and information asymmetry and traces its history, debates, and criticisms. These concepts are then applied to key themes in public administration – performance management, federalism/decentralization, contracting, politics-administration, and institutional drift. Part II investigates how these problems manifest and tackled in the US, China, and Europe. Part III concludes with a synthesis of findings, debates, extensions, and future directions for theory and practice.