The United States Constitution protects the free exercise of religion, but does not define what religion is. Religious liberty in the United States, however, hinges on the meaning of this term in the First Amendment. This dissertation describes and analyzes how the Supreme Court has conceptualized religion in its free exercise jurisprudence. Based on the analysis of free exercise cases decided between 1878 and 2005, this study finds that justices have offered three different accounts of religion. It classifies these as ontological, sociological, and individualistic approaches—depicting religion with reference to truth, groups, and individuals, respectively. The Supreme Court’s earliest free exercise decisions address religion ontologically, defining it in terms of metaphysical truth. The Court’s subsequent efforts to avoid ontology result in two distinct lines of religion decisions. Each of these reflects an alternative definitional method—one individualistic, one sociological. The individualistic approach construes religion in broad and indefinite terms, avoiding Establishment Clause conflicts by excluding very little from the category of “religion.” The sociological mode proves narrower, appealing to the characteristics of recognized religious groups to identify and protect religious conduct. While the present state of religion jurisprudence remains convoluted, this dissertation offers three conclusions. First, the three-fold model for how justices think about religion proves a useful tool for describing and analyzing Supreme Court opinions regarding religious exercise. Second, applied to the history of free exercise jurisprudence, this model helps to explain variations in the Court’s decisions. Third, while the Supreme Court’s current individualistic approach provides relatively little protection to religion, returning to a more ontological mode offers some promise.