Ethical concepts are, or purport to be, normative. They make claims on us: they command, oblige, recommend, or guide. But where does their authority over us come from? Christine Korsgaard identifies and examines four accounts of the source of normativity that have been advocated by modern moral philosophers--voluntarism, realism, reflective endorsement, and the appeal to autonomy--and shows how Kant's autonomy-based account emerges as a synthesis of the other three. Her discussion is followed by commentary from G.A. Cohen, Raymond Geuss, Thomas Nagel, and Bernard Williams, and a reply by Korsgaard.