Peremptory norms, or norms jus cogens, hold a unique position in the hierarchy of international law. Unlike customary international law and treaty law, peremptory norms abide no derivation and are binding on all states regardless of their willingness to be bound by them. As a result, the authority of peremptory norms, it is argued, cannot be adequately explained by current positivist and voluntarist explanations of their authority. This article discusses the inadequacies of the positivist explanation and puts forward an alternative natural law explanation for the authority of peremptory norms which avoids the conceptual difficulties found in the positivist account. Finally, in the concluding section I address a number of potential realist and post-modernist counter arguments to my position and dismiss them as unconvincing.