Crimes against humanity were first defined, for the purposes of the Nuremberg Trial, in 1945. Since then, numerous international legal texts have incorporated the concept, the latest being the Rome Statute of the ICC, 1998. The different texts offer diverse definitions of crimes against humanity, which are traced in the article. Although the precise outlines of the crimes change from one definition to another, it is clear that the core has crystallized as an integral part of customary international law. In the Tadić case, the ICTY had to address several crucial issues relating to crimes against humanity. The judgments on appeal will serve as precedents for the removal of the linkage between crimes against humanity and armed conflict, the exclusion of isolated attacks against civilians and the irrelevance of the personal motives of the defendant.