It is widely accepted that there is a jus cogens norm in the jus ad bellum. Yet uncertainty exists as to its scope, due to the existence of two exceptions to the prohibition on the use of force: self-defence and force authorised under the UN Charter. This dissertation aims to identify the jus cogens norm in the jus ad bellum by clarifying the meaning of ‘derogation’ in the context of jus cogens norms. In doing so, this dissertation addresses three questions: 1) what does it mean to say that a norm permits no derogation?; 2) do exceptions derogate from jus cogens norms?; and 3) how do the exceptions to the prohibition on the use of force affect the scope of the jus cogens norm in the jus ad bellum? A jus cogens norm is identified as a norm of general international law ‘accepted and recognised by the international community of States as a whole as a norm from which no derogation is permitted’. Yet scholars understand ‘derogation’ in different ways: a ‘narrow’ view understands derogations as attempts to contract out of the jus cogens norm inter se, while a ‘broad’ view understands derogations as any norm that makes conduct inconsistent with the jus cogens norm lawful. On the broad view, there can be no exceptions to jus cogens norms. It is argued that to say a norm permits no derogation means that there can be no lawful conduct inconsistent with obligations arising under that norm. As a result, any apparent ‘exceptions’ to jus cogens norms must be limitations on the scope of that norm. Thus, neither force lawfully authorised under the Charter nor force lawfully used in self-defence is prohibited by the jus cogens norm in the jus ad bellum. Such uses of force are defined and regulated by non-jus cogens norms, which are recognised by the jus cogens norm without forming part of it.