Contrary to stylized accounts of policy making in democracies, it is routine for presidents, governors, and other chief executives to issue directives such as decrees and executive orders to make law on their own. This article evaluates what political scientists have learned about presidential unilateral power. In our view, while a quarter century of scholarship on the topic has yielded a variety of theoretical predictions, the empirical record offers conflicting and perhaps unreliable evidence to substantiate and adjudicate between them. We review the dominant theoretical perspectives, which focus largely on constraints related to the separation of powers and political accountability. We then evaluate the evidence supporting these arguments and conclude with recommendations for conceptual, theoretical, and empirical advancement. Expected final online publication date for the Annual Review of Political Science, Volume 24 is May 11, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.