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—something that is relatively uncontroversial in the secondary literature. The second section examines key aspects of reason in the moral philosophy, with especial 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, emphasizing especially the most prominent contemporary interpretation of Kantian reason, that of Onora O'Neill. The concluding remarks underline the potential philosophical interest of such a unified interpretation of Kant's account of reason.