Emil Stanisław Rappaport took part in the major international debates concerning criminalization of international crimes and at the same time he had an opportunity to introduce all discussed concepts in law of the reborn state. The first part of the chapter focuses on the early years of Rappaport’s academic career and the influence on him of the sociological school of criminal law. Rappaport’s work as a defence attorney and its impact on his further research on criminal enforcement law is discussed. The second part is devoted to Rappaport’s contribution to the development of a legal system and court system in the newly independent Republic of Poland (he considered the Codification Committee as a laboratory of modern legislation, a belief shared by the Association internationale de droit pénal). The third part presents Rappaport’s efforts in various international associations of criminal lawyers, and his work towards the unification of criminal laws worldwide and towards the establishment of the foundations of international criminal law. The final part discusses the period during and after World War II, when Rappaport had to face the challenges of bringing Nazi criminals to justice while dealing with the spread of Soviet influence across the Polish judiciary and academia.