In this paper I will argue that, international criminal law constitutes a valid legal system. There is skepticism over the authority of international law; some claim that it is not a genuine legal system, and does not command legal obligation. I will adopt a particular legal positivist position, known as moral attitude positivism, and apply this to an analysis of international criminal law. I will argue that the criteria necessary for a legal system on the positivist account are present in the international system of criminal law. Specifically, I will argue that a ‘rule of recognition’ can be drawn from a variation of the legal norm pacta sunt servanda, and this serves to validate the system as a whole. I conclude that the skeptic is wrong, and that international criminal law should be regarded as a valid legal system, commanding genuine legal authority.