Abstract: Legitimacy is a term much invoked but little analyzed in constitutional debates. Uncertainty and confusion frequently result. This Article fills a gap in the literature by analyzing the idea of constitutional legitimacy. It argues that the term invites appeal to three distinct kinds of criteria that in turn support three distinct but partly overlapping concepts of legitimacy - legal, sociological, and moral. When we examine legitimacy debates with these three concepts in mind, striking conclusions emerge. First, the legal legitimacy of the Constitution depends more on its present sociological acceptance than on the (questionable) legality of its formal ratification. Second, although the Constitution deserves to be recognized as morally legitimate, it is only "minimally" rather than "ideally" so: it is not morally perfect, nor has it ever enjoyed unanimous consent. Third, because the Constitution invites disagreement about what it means and how it should be interpreted, many claims about the legal legitimacy of practices under the Constitution rest on inherently uncertain foundations. Significantly, however, a virtual consensus exists that at least some judicial precedents suffice to support future claims of legitimate judicial authority, even when those precedents were themselves erroneously decided in the first instance. Like the legal legitimacy of the Constitution, the legal legitimacy of precedent-based decisionmaking arises from sociological acceptance. Fourth, in the absence of greater legal and sociological consensus, judgments about many purportedly legal questions, including questions of judicial legitimacy, frequently reflect assumptions about the moral legitimacy of official action. Realistic discourse about constitutional legitimacy must therefore reckon with the snarled interconnections among constitutional law, its sociological foundations, and the felt imperatives of practical exigency and moral right.