Abstract: The nature of the presidency cannot be understood without reference to norms. The written provisions of our constitutional structure do not, by themselves, offer a sufficiently thick network of understandings to create a workable government. Rather, those understandings are supplied by norm-governed practices. Presidential power is both augmented and constrained by these unwritten rules. The article offers a sustained account of the norm-based presidency. It maps out the types of norms that structure the presidency, and excavates the constitutional functions that these norms serve, the substantive commitments that they supply, the decisional arenas where they apply, and the conditions that make some norms (relative to others) more or less fragile. Understanding these characteristics of an unwritten Article II helps to mark abnormal presidential behavior when it arises. It also brings into view core features of structural constitutionalism itself. Norms simultaneously settle constitutional duty for a time and orient contestation over what legitimate practice should be. Norms, however, cannot be understood in contrast to a fixed constitutional structure. Rather, norms bring into view the provisional nature of our constitutional order itself. The role of presidential norms in constituting a working government raises a pressing question for public law theory: What happens when these norms break down—when the extralegal system ceases to enforce them? The article sketches a spectrum of judicial responses, each of which finds occasional (though often implicit) support in the case law. Prescriptively, it argues that when the norms of the presidency collapse, the norms of the judiciary appropriately adjust. Underlying judicial deference is an antecedent question of institutional choice: should the court or the president decide the question at issue? The court’s answer to that question is predicated on (sometimes unarticulated) institutional assumptions about how the presidency actually functions—that the presidency is governed by norms that restrain self-dealing or promote considered judgment. Absent such norms, the court is confronted with a very different institutional choice; a court that ignores these presidential norms decides the legal question on false premises. Courts, however, are and should remain limited players in a norm-based constitutional order. Norms often do not implicate an independent and judicially enforceable legal claim. And the more society depends on courts to check norm breaching by political actors, the more fragile norms of the judiciary may become. Ultimately, it is extra-judicial institutions that sustain or erode the norm-based features of the presidency, and of American constitutional democracy.