Hans Kelsen’s interventions in the law of the Nuremberg trials have remained unexplored with the exception of notes in passim or a couple of brief studies. International criminal law, is true, was not Kelsen’s main object of study. However, the events culminating in Nuremberg, both the shocking Nazi policies and that Kelsen was an émigré in the United States were instrumental to precipitate his close engagement with international criminal law during the dawn years between 1942 and 1945. This chapter introduces the man and his ideas, traces elements of the pure theory that relate to the principle of individual criminal responsibility and, with the help of hitherto unknown archival material, describes for the first time Kelsen’s personal participation in the preparations of Nuremberg and the history behind his absence from the International Military Tribunal. The chapter concludes with a selection of two aspects of Kelsen’s activities as a legal advisor for the US War Crimes Commission: first, his insistence on the need for an international treaty as the legal basis of the IMT in order to avoid victors’ justice and problems of jurisdiction; second, his contribution towards the inclusion of individual criminal responsibility in the London Agreement.