This chapter introduces genocide as both a legal concept and one that remains contested. It begins with the 1948 adoption of the United Nations Convention on the Prevention and Punishment of the Crime of Genocide (Genocide Convention), which defines genocide as specific acts committed with intent to destroy national, ethnic, racial, or religious groups. The chapter traces the origins of the term to Raphael Lemkin, who coined “genocide” and understood it not only as mass killing, but as a coordinated attack on the physical, biological, and cultural foundations of group life. The chapter then follows the political process that transformed Lemkin’s expansive concept into the one codified in international law. Early drafts of the Genocide Convention included protection for political groups and acts of cultural genocide, but these were removed through state negotiations. As adopted, the Genocide Convention narrows genocide largely to physical and biological destruction. Finally, the chapter surveys ongoing scholarly debates concerning group protection, cultural genocide, genocidal intent, and anthropocentrism and speciesism.