Abstract: This Essay identifies and analyzes the President's completion power: the President's authority to prescribe incidental details needed to carry into execution a legislative scheme, even in the absence of congressional authorization to complete that scheme. The Essay shows that the completion power is a common explanation for very different presidential powers, including the administration of a presidential statute, prosecutorial discretion, and the use of force abroad without express congressional authorization. Maintaining that the widespread use of the completion power is a partial vindication of Chief Justice Vinson's neglected dissent in the Youngstown Steel Seizure case, this Essay argues that the completion power sheds light on a structural symmetry that cuts across Articles I, II, and III of the Constitution--namely, that each of the three branches has some degree of inherent power to carry into execution the powers conferred upon it. The Essay also examines normative questions about the scope and limits of the power.