The chapter gives an account of genocide, linking this with the wider phenomenon of state sponsored killing. Firstly some of the key dimensions of genocide in legal and extra-legal scholarship are introduced, identifying points where differing interpretations of the crime emerge. The prevalence and distribution of genocide is examined in terms of a snapshot of the current situation, historical surveys and attempts to calculate the number dying in state sponsored killings. An account of different ways of thinking about victims and perpetrators leads in to a summary of explanatory frameworks, concluding with recent attempts to create multi-level, integrated explanations for genocide. Finally, the record of preventive action since 1948 is examined, suggesting some reasons for changes in the willingness to intervene during, or to prosecute after, genocidal episodes.