This lecture examines the mechanisms by which war can be prevented in international relations. What works, what does not, and why? Theoretical notions and empirical findings will be introduced to address three main sets of issues: first, how do states negotiate or act to prevent the escalation of their disputes into war? Should they really prepare for war if they want peace? Can appeasement work? And do mediators help? Second, if negotiations fail, can long and costly wars be avoided - that is, how can war be terminated most effectively? Does peace-making work? Finally, once peace has been obtained, what mechanisms work best to maintain it? When is peace-keeping effective? And how should agreements be crafted to avoid the recurrence of war? These questions will be addressed in the context of both civil and interstate wars.