This Chapter traces the historical developments of the criminalization of aggression, and whether acts of aggression might be described as an international crime. The Chapter first discusses the Cambridge Commission, followed by the first serious debates and elaborate studies concerning the prosecution of aggression or crimes against peace at the London International Assembly, then the United Nations War Crimes Commission, and concludes with the London Conference. The uncertainty about the role of aggression within the overall system of international criminal law is not only characteristic of the debate that immediately preceded Nuremberg, but it is also manifested in the approach to the issue in the decades that were to follow the landmark trial. The Chapter concludes by drawing parallels between the difficulties faced by the United Nations War Crimes Commission in taking a position on whether or not aggressive war should be a crime with the indecision at the Rome Conference on including aggression. While ultimately aggression was included, the International Criminal Court is still unable to exercise jurisdiction over the crime. Thus, the work that began in Paris and continued in London, then Nuremberg, and later in Rome and Kampala, remains unfinished and incomplete.