The aim of this dissertation is to follow the development of Kants theory of freedom from his discussion in The New Elucidation of the First Principles of Metaphysical Cognition (1755) to the theory of transcendental freedom presented in the solution to the Third Antinomy in the Critique of Pure Reason (1781, 1787). In this earlier work, Kant defends a compatibilist theory of freedom which, contrary to traditional interpretations, explicitly rejects the ability to do otherwise as a necessary condition for free agency. This is a result of his acceptance of the Principle of Sufficient Reason. By the time of the Critique, Kant had come to reject this view in favor of an incompatibilist theory of transcendental freedom. At the same time, the Principle of Sufficient Reason had become a mere regulative principle intended to help organize our investigations. The problem here is threefold: 1) to explain why Kant came to reject compatibilism, 2) to explain why Kant came to regard the Principle of Sufficient Reason as a regulative, rather than a substantive, principle, and 3) to explain why, given the cosmological nature of the antinomy, Kant expresses its solution in largely moral terms. These issues are intimately connected. I contend that these changes in Kants views are the result of his growing awareness during this period of the fundamental tension between the compatibilism required by his rationalist metaphysics and the incompatibilism of his ethics. In light of this, I argue that the Third Antinomy is best understood as an attempt to resolve this tension by demonstrating that the legitimate regulative use of the Principle of Sufficient Reason does not challenge our status as moral agents.