UN peacekeeping serves as the foremost international tool for conflict intervention and peace management. Since the Cold War, these efforts have almost exclusively targeted conflicts within, rather than between, states. Where traditional peacekeeping missions sought to separate combatants and monitor peace processes across state borders, modern peacekeeping in civil wars involves a range of tasks from intervening directly in active conflicts to rebuilding political institutions and societies after the fighting ends. To accommodate this substantial change, peacekeeping operations have grown in number, size, and scope of mandate. The increasing presence and changing nature of peacekeeping has sparked great interest in understanding when and how peacekeeping is used and how effective it is in delivering and sustaining peace. Significant advances in peacekeeping data collection have allowed for a more rigorous investigation of the phenomenon, including differentiation in the objectives, tasks, and structure of a mission as well as disaggregation of the activities and impact of peacekeepers’ presence across time and space. Researchers are particularly interested in understanding the adaption of peacekeeping to the unique challenges of the civil war setting, such as intervention in active conflicts, the greater involvement and victimization of civilians, the reintegration of rebel fighters into society, and the establishment of durable political, economic, and social institutions after the fighting ends. Additional inquiries consider why the UN deploys peacekeeping to some wars and not others, how and why operations differ from one another, and how the presence of and variation across missions impacts conflict countries before and after the fighting has stopped.