Numerous scientific works bear witness to the ambivalence of the concept of crime against humanity; works in which the main objective is to respond to the "how". The present study offers to respond to the "why" and to reconstitute the history of the concept through primary sources as well as through the abundant preexisting doctrine, and to look back to the concept's beginnings while adopting new viewpoints. This original approach requires rendering the legal factors determining the evolution of the concept of crime against humanity visible, demonstrable and explicit. Moreover, the analysis rests on those works devoted, more generally, to age-old questions raised by the internationalization of criminal law and to the emergence of an international criminal legal order, inseparable from that of the concept and from the ever-changing nature of current events.