In recent years, the requirement that administrative agencies provide adequate reasons for their decisions has come to play a central role in judicial review of agency decisions. While the increasing importance of this requirement has been recognized, no systematic study of its history and doctrinal basis has been undertaken. Grounded in a survey of court of appeals decisions reviewing agency action, and a careful review of Supreme Court decisions, this article proposes that the requirement is best understood as a form of heightened scrutiny of the rationale of agency decisions and that the doctrine of separation of powers requires such scrutiny because of the unique position of administrative agencies in terms of the constitutional structure of government. The separation of powers conception of the reasons requirement derives from an analysis of political values underlying administrative law, the evolution of various models of judicial review, and the history of the reasons requirement itself.The article begins by considering the political values underlying our governmental structure and describes the apparent conflict between two competing sets of values. ‘Liberal’ values restrict government action in order to preserve individual freedoms and are reflected in the Constitution through principles such as representative government, separation of powers, and due process. ‘Progressive’ values promote government action in order to relieve social problems, and are implemented through delegation of legislative and judicial powers to unelected administrators functioning outside of the political and constitutional limitations originally established for the exercise of those powers. Thus, administrative law jurisprudence is faced with the difficult task of accommodating two sets of conflicting values.The article then examines the Supreme Court’s efforts to accommodate liberal and progressive values through its articulation of the scope of judicial review of agency decisions. This examination identifies three distinct models of review reflecting the Court’s evolving jurisprudence—a ‘structuralist,’ a ‘proceduralist’ and a ‘rationalist’ model. We argue that the structuralist and proceduralist models have failed to accommodate progressive and liberal values both as a theoretical and practical matter. These models could not explain how liberal values had been preserved in the face of acceptance of progressive programs; nor could they provide meaningful protection for affected parties without unduly impairing the administrative process. We suggest that as a result the Court is turning to the rationalist model of judicial review, which has as its central feature the requirement that agencies articulate adequate reasons for their decisions. However, the Court has not fully explained the significance of this requirement, or the doctrinal basis of the rationalist model.The article locates that doctrinal basis by tracing the heretofore unexplored origin and development of the adequate reasons requirement. This history reveals that the requirement originally had a separation of powers dimension which was later obscured by the influence of the proceduralist model. This separation of powers dimension provides the basis for a more complete articulation of the rationalist model of judicial review under which the requirement of adequate reasons is best understood as a product of the separation of powers doctrine. This understanding not only reflects the history of the requirement, but also enables the rationalist model to accommodate liberal and progressive values. At the theoretical level, the rationalist model explains how structural safeguards are preserved so as to protect liberal values. At the practical level, the rationalist model provides a meaningful check on administrative action without unduly impairing the implementation of progressive programs by administrative agencies.