At the core of Kant's Groundwork of the Metaphysics of Morals lies his ‘derivation’ of the categorical imperative: his attempt to establish that, if there is a supreme principle of morality, then it is this imperative. Kant's argument for this claim is one of the most puzzling in his corpus. The received view, championed by Aune and Allison, is that there is a fundamental gap in the argument, which Kant elides by means of a simple but deadly confusion, thus robbing the argument of all validity. We will here contest the received view, as well as Korsgaard's alternative interpretation of the argument. In place of these positions we will offer a reconstruction of the derivation which reveals its coherence and force. We will show that it illuminates some interesting grounds for rejecting certain candidates, including a utilitarian principle, for status as the supreme principle of morality. While certainly not free of all defects, the argument will be shown to be far more powerful and interesting than it has commonly been held to be.