The scope of the President’s legal authority is determined in part by historical practice. This Essay aims to better understand how such practice-based law might operate as a constraint on the presidency. Some scholars have suggested that presidential authority has become “unbounded” by law, and is now governed only or primarily by politics. At the same time, there has been growing skepticism about the ability of the familiar political checks on presidential power to work in any systematic or reliable fashion. Skepticism about law’s potential to constrain in this context is heightened by the customary nature of much of what purports to be the law of presidential power, and by the limited availability of judicial review. As the Essay explains, no examination of whether law constrains the President can succeed without careful specification of what constraint entails and how it relates to distinct but related phenomena like genuine disagreement about the content of the law. After attempting such specification, the Essay identifies various internal and external causal mechanisms through which law, including practice-based law, could constrain the President. The Essay explains, among other things, that one way that law might constrain the President is through the simple fact that issues of presidential power are publicly criticized and defended in legal terms. To gain additional traction on the topic, the Essay considers two other areas of debate that pose analogous questions about the constraining effect of law: whether the doctrine of stare decisis (itself a practice-based norm) constrains Supreme Court decisionmaking, and whether international law (including the international law that is based on customary practice) constrains the behavior of nation-states. The Essay concludes by suggesting some avenues of possible empirical research.