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. In part because of the limited availability of judicial review in this area, some commentators 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. Whether and how practice based law might constrain the President are thus vital questions. As the Essay explains, no examination of those questions can succeed without careful specification of what legal 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 argues, among other things, that one way that law might operate as a constraint is through the simple fact that issues of presidential power are publicly criticized and defended in legal terms. The Essay concludes by suggesting some avenues of possible empirical research.