in Thomas G. Weiss and Sam Dawes, eds., The Oxford Handbook of the United Nations, 2nd ed. (Oxford University Press, 2018). Peacebuilding – helping societies make the transition from civil violence to a durable peace – has been the United Nations’ principal security activity since the end of the Cold War. Although peacebuilding methods have been refined during years of trial and error, it remains an uncertain science, yielding mixed results. But for all its shortcomings, the international peacebuilding "project" remains one of the most remarkable exercises in collective conflict management the world has ever witnessed. This chapter identifies the principal features of the UN’s peacebuilding operations, examines the record of peacebuilding since the end of the Cold War, and describes some of the main issues and controversies surrounding these missions.