In this chapter, I examine whether punishment is morally permissible. Criminal prosecution and punishment are the main functions of international criminal justice and lie at the heart of what institutions of international criminal justice – such as the International Criminal Court – do. To begin, I reconstruct Locke’s argument for the permissibility of punishment. Locke argues that we have a moral duty to ‘preserve humanity’ and we can discharge that duty by exercising our ‘natural executive right’ – that is, a right to punish – in the state of nature. Punishments are effective in enforcing rights because they deter crimes. I discuss several objections against the consequentialist structure of Locke’s argument. To counter these objections, I argue for a mixed theory that distinguishes between justifying the practice of punishment and justifying the distribution of punishment within that practice. To explain the latter, we must take into account considerations of normative individualism, egalitarianism, and pragmatic considerations. Taken together, these arguments imply that coercive punishment is permissible for natural rights enforcement.