From the Nazi concentration camps to the killing fields of Pol Pot’s Cambodia to the deserts of Darfur, human beings have displayed a seemingly unlimited capacity for cruelty. International legal prohibitions on war crimes, crimes against humanity, and genocide are designed to deter and punish such crimes. Although treaties regulating the wartime conduct of soldiers and states predated the formation of the International Military Tribunal at Nuremberg, the tribunal was the first international body ever established to try state leaders, not only for wartime conduct but also for their domestic policies. We examine the complex legal issues associated with the prosecution of these crimes by tracking the development of international institutions from Nuremberg to the International Criminal Court.