The category of crimes against humanity is one of the most often used qualifications to present crimes committed at the international scale. International organizations, major nongovernmental organizations (NGOs), diplomats, judges, lawyers, and scholars all use this useful concept as an authoritative one. But the definition of crimes against humanity is far less clear than what its overuse leads us to believe. This entry examines the development and recognition of the concept of crimes against humanity and discusses its evolving definition and application in the arena of international justice from the Nuremberg Tribunal to the International Criminal Court (ICC).