Abstract: Under Marbury v. Madison, it is emphatically the province and duty of the judicial department to say what the law is. But as a matter of actual practice, statements about what the law is are often made by the executive department, not the judiciary. In the last quarter-century, the Supreme Court has legitimated the executive's power of interpretation, above all in Chevron v. Natural Resources Defense Council, the most-cited case in modern public law. Chevron reflects a salutary appreciation of the fact that the executive is in the best position to make the judgments of policy and principle on which resolution of statutory ambiguities often depends. But the theory that underlies Chevron remains poorly understood, and in the last two decades, significant efforts have been made to limit the executive's interpretive authority. In general, these efforts should be resisted. The principal qualification involves certain sensitive issues, most importantly those involving constitutional rights. When such matters are involved, Congress should be required to speak unambiguously; executive interpretation of statutory ambiguities is not sufficient.