This dissertation seeks to answer, by close reading of primary texts, the question of whether John Calvin and Thomas Aquinas share a common theory of natural law. Calvin and Aquinas both use a common vocabulary of natural law inherited from antiquity. They both speak of a morally normative natural order, and appeal to nature as a morally evaluative standard when discussing practical questions. While they share a common vocabulary of natural law, however, it is not clear whether this is undergirded by a common set of theological assumptions, or whether Calvin and Aquinas have different visions of natural law, reflecting major theological differences between Reformed and Roman Catholic Christianity. Existing literature raises, but does not adequately explore, questions concerning the compatibility of Calvin's thought on natural law with that of Aquinas. There currently exists no book-length study of Calvin's thought on natural law, but by comparing Calvin with Aquinas, rather than studying him alone, I hope the features of his approach can be more easily identified. The contention of this thesis is that Calvin and Aquinas do share a common vision of natural law. To establish this conclusion, I examine areas of apparent disagreement between Aquinas and Calvin that I have identified by reviewing existing comparative literature. These disagreements concern the role that natural law plays in systematic theology, the relationship between natural law and conscience, the relationship between natural law and the Decalogue, the noetic effects of sin, and the moral status of natural inclinations. In each case, I argue, apparent differences mask a deeper agreement between Aquinas and Calvin that becomes clearer when a broader range of primary texts are subjected to closer scrutiny than has hitherto been the case.