Professor Aron Trainin (1883–1957), an outstanding Russian and Soviet criminal law scholar, has played a leading role in academic support of the Soviet Union’s team in Nuremberg and contributed to the drafting of the IMT Charter and Judgment. His writings to a large extent shaped the Soviet approach to international criminal law. The chapter addresses Trainin’s biography and follows the major steps in his academic career from its beginning in tsarist Russia to its the peak, when Trainin held the positions of associate member of Soviet Academy of Sciences and professor at Moscow University. The chapter traces the sources of Trainin’s interest in international criminal law and addresses the scholar’s contribution to the ICL. The author focuses specifically on two areas, where the impact of Trainin’s was significant – a legal justification for crimes against peace, and the concept of complicity.