Are United Nations peacekeeping missions effective at reducing violence in civil wars? Although UN peacekeeping is a notable intervention tool, the international community lacks systematic knowledge of how well it mitigates civil war violence. Given that UN peacekeeping is increasingly used in the midst of war, this is a significant research gap with direct policy relevance. This book systematically explores if and how the capacity and constitution of UN peacekeeping missions affect the amount of violence in civil conflicts. It argues that peacekeeping effectiveness needs to be assessed in relative terms, theorizing that more robust missions are increasingly capable of addressing combatant incentives for employing violence. The authors conduct large-n analyses of the number of combatants and civilians killed during each month for all civil wars globally from 1992 to 2014, measuring the capacity and constitution of UN missions with unique data on the number and type of peacekeeping personnel deployed. The analyses reveal that increasing UN military troop and police personnel deployed to a conflict significantly reduces violence against civilians, and increasing UN military troop personnel significantly mitigates battle-related violence. By contrast, smaller missions and missions composed of observers are not associated with reduced violence. The book complements the large-n analyses with qualitative explorations of peacekeeping mechanisms on violence in Côte d’Ivoire and the Democratic Republic of Congo. The authors conclude that while peacekeeping is not without detriments, it is an effective tool of violence reduction.