An enduring challenge for administrative law is the tension between the ideal of democratic policymaking and the ubiquity of bureaucratic discretion. This Article seeks to reframe the problem of agency discretion by outlining an interpretivist model of administrative law based on the concept of fiduciary obligation in private legal relations such as agency, trust, and corporation. Administrative law, like private fiduciary law, increasingly relies upon a tripartite framework of entrustment, residual control, and fiduciary duty to demarcate a domain of bounded agency discretion. To minimize the risk that agencies will abuse their entrusted discretion through opportunism or carelessness, administrative law empowers the political branches to exert limited residual control over agencies and subjects agencies to nonderogable duties of care and loyalty. As an interpretivist theory, this fiduciary model helps to explain controversial features of administrative law such as the contemporary nondelegation doctrine, Chevron deference, and the limits of presidential control over agency action. By clarifying administrative law's internal dynamics and implicit ambitions, the fiduciary model also provides a blueprint for reform in critical areas such as the standing doctrine and the due process restraints on agency discretion.