Criminology has largely ignored the study of crimes against humanity even though the acts involved—genocide, murder, rape, torture, the appropriation or destruction of property and the displacement and enslavement of populations—are criminal under national and international law and more serious than most crimes commonly studied by criminologists. We examine why criminology has neglected these crimes, argue that criminological theorizing will benefit by attending to this substantive area and put forward a theory of crimes against humanity derived from and expanding on existing criminological theory both to offer a basis for new theoretical and empirical work and to illustrate how criminological theories might be modified to provide more powerful accounts of crime. The article draws on a case example of genocidal mass-murder: Jedwabne, Poland, July 1941.