This chapter examines theories that explain the causes of war. It considers ideas advanced by political scientists, sociologists, biologists, and philosophers, showing that different explanations of war give rise to different requirements or conditions for peace. After highlighting the difficulties in studying war, the chapter discusses human nature explanations of war, citing such factors as frustration, misperception, misunderstanding, miscalculation, and errors of judgement as well as the role of human collectives including factions, tribes, nations, and states. It then describes the bargaining model of war before turning to inter-state wars, intra-state conflicts, and ethnic conflicts. It also explores the debate over whether ‘greed’ or ‘grievance’ are the main causes of civil wars. The chapter concludes that identifying a single cause appropriate to all wars is an exercise in futility and that a worldwide ‘just’ peace is unattainable.