AbstractThis article argues that if there is a jus cogens norm in the jus ad bellum, it must be the customary norm which prohibits non-consensual uses of force that are neither validly authorised under the UN Charter nor lawful exercises of self-defence. In doing so this article will clarify the method by which jus cogens norms should be identified, based on a correct understanding and application of what it means for a norm to be ‘accepted and recognized as a norm from which no derogation is permitted’. It is argued that all existing jus cogens norms must be norms of customary law, and that the uncertainty regarding the scope of the jus cogens norm prohibiting force results from uncertainty as to the structure of the underlying customary and treaty law norms in the jus ad bellum.