On 24 May 1915, the Triple Entente powers of Britain, France, and Russia issued a joint declaration that qualified the massacres, imprisonments and violence inflicted on the peaceful Armenian population as “crimes against humanity and civilization.” The concept of crimes against humanity was further developed and explained by the Commission on the Responsibility of the Authors of the War and on Enforcement of Penalties (hereinafter: Commission), which was formed during the Paris Peace Conference to investigate violations of the laws and customs of war and prosecute war criminals. This article traces the evolution of discussions held within the Commission regarding the atrocities committed by the Ottoman Turkish government against its Armenian population, which technically fell outside the jurisdiction of the crimes specified in these talks, as the atrocities were committed not against the population of a belligerent state but, rather, against their own nationals. These discussions played a major part in the development of international criminal law; particularly of the concept of crimes against humanity. The Armenian Memorandum and the list of perpetrators of the Armenian atrocities were also presented to the Commission, which (together with other data and information collected) identified the crimes committed against the Armenian population by the Ottoman authorities, thus defining the concept of crimes against humanity and the acts that fell within its scope. Although post-World War I efforts for justice are generally assessed as a failed effort, crimes against humanity first materialized as a term in international law during this period and its development was based on the Armenian atrocities, which provided a critical basis for the International Military Tribunal's later convictions and contributed to the concept's evolution into its present-day significance.