The Supreme Court’s willingness to defer to agency interpretations of ambiguous statutes has vacillated over the past seventy years. The Court’s vacillation has dramatically impacted the executive’s power to make and interpret law. This Article examines how the Court augmented then constricted executive lawmaking power and ceded then reclaimed executive interpretive power with a single case and its legal progeny. Chevron U.S.A., Inc. v. Natural Resources Defense Council, Inc.1 and its aftermath dramatically altered the executive’s power to make and interpret law. Prior to Chevron, Congress had the primary responsibility for lawmaking, while agencies made policy choices primarily when Congress explicitly delegated that power to them. Also, p...