Only the innocent deserve the benefits of the principle of legality. This assertion naturally offends our notions ofjustice. It would be unacceptable for a legal system to institutionalize such an approach. Yet, in the context of prosecuting mass atrocities, genocide, crimes against humanity, and war crimes, international criminal justice mechanisms appear to be resigned to such a principle, if not openly embracing it. Although ranking among the most fundamental principles of criminal law, nulla poena sine lege (no punishment without law) receives surprisingly little attention in international criminal justice. Indeed, it may be considered the "poor cousin" of nullum crimen sine lege (no crime without law), which has attracted far greater consideration. Whereas nullum crimen addresses the punishability of the conduct in question, nulla poena deals with the legality of the actual punishment or penalty itself Given that both are at the core of the principle of legality, the neglect of nulla poena sine lege is difficult to justify, although not without explanation. As one prominent scholar observes, nulla poena "affects only proven criminals" while nullurn crimen "protects the mass of respectable citizens." While most criminal justice systems have made considerable efforts to close this gap over the years, international criminal justice has not. The potential contribution of nulla poena sine lege has been overlooked on the international level by policy makers, drafters, and judges. Likewise, there exists a lacuna in academic scholarship on this subject. Under-theorization of nulla poena in international criminal justice stalls the maturation in international law of this longstanding criminal law principle, keeps dormant its contribution to justice, and challenges the legitimacy of international punishment. This Article aims to redress this imbalance by (1) developing the normative content of nulla poena sine lege under international law; (2) critically evaluating the statutes of international criminal courts and their sentencing jurisprudence on genocide, crimes against humanity, and war crimes; and (3) advancing a theory for understanding the role and potential contribution of nulla poena to international justice. I argue for an understanding of nulla poena that goes beyond its simple caricature as a principle of negative rights, designed merely to prevent retroactive punishment, to one that captures its full contribution to justice, including equality before the law, consistency in punishment, and legitimacy in international prosecutions. By advancing an international standard for nulla poena sine lege, I hope to lay a foundation on which international sentencing can more readily achieve the goals of the international community in prosecuting and punishing perpetrators of mass atrocities.