 This entry has the following structure. The first section sets out the role that reason plays in Kant’s account of knowledge and metaphysics in the first Critique. The second section examines key aspects of reason in the moral philosophy, with special reference to the second Critique. Reflecting Kant’s canonical texts and the bulk of the secondary literature, these discussions of theoretical and practical reason are relatively independent of one another. The third section therefore considers how Kant’s views of theoretical and practical reason may be related. It emphasizes the most prominent contemporary interpretation of Kantian reason, that of Onora O’Neill. The concluding remarks underline the philosophical interest of a unified interpretation of Kant’s account of reason.