This thesis provides an analysis of the covenant thought of Zacharias Ursinus (1534- 1583) who further advanced the idea of a bi-covenantal scheme in the development of covenant theology in the sixteenth century and seeks to demonstrate how such advancement did not diminish the gratuitous nature of the gospel. Understood within the larger framework of his theological system it becomes evident that his original foedus naturale did not arise out of an overly speculative scholastic methodology. Instead, it served as the corollary of the foedus gratiae he inherited from his Reformed predecessors. An analysis of his teaching on the natural law, the law-gospel dichotomy, the federal headship of Adam and Christ and the imputation of Christ’s righteousness for Adam’s disobedience reveals how the foedus naturale emerged organically from his convictions on these heads of doctrine. The methodology is both systematic and historical. It begins with a study of his theological development as examined in the context of his life and written works followed by a review of the contributions of previous secondary literature that exists on this subject. Both the doctrine of God and the doctrine of the knowledge of God are analyzed as they provide insight into his theological methodology showing it to be biblical rather than philosophically speculative. The heart of the thesis is a close examination of the foedus gratiae and the foedus naturale which demonstrates how the two complement each other and how they relate to other theological distinctives within his thought. The study concludes by showing how his doctrine of justification, namely double imputation, lays the basis for the formulation of his pioneering idea of a foedus naturale.