This essay explains Kant’s idea of autonomy of the will and advances a thesis about how it emerges in his moral conception. Kant defines “autonomy” as “the property of the will by which it is a law to itself…” and argues that the Categorical Imperative is that law. I take the autonomy of the will to mean that the nature of rational volition is the source of the formal principle that authoritatively governs rational volition. I give a sense to this idea by pointing to an argument form found throughout the Groundwork and the second Critique where Kant moves from a conception of rational volition as a faculty to a statement of its formal principle. This idea of autonomy emerges in Kant’s moral conception (at the time he writes the Groundwork) as his solution to the problem of moral theory. Common sense assumes that moral requirements apply with unconditional necessity, and the problem of moral theory is to show how such requirements are possible. Kant’s resolution to this problem is to argue that the necessity of moral requirement is genuine only if based in autonomy of the will, only if based in a “law that arises from one’s will”.