Does United Nations peacekeeping protect civilians in civil war? Civilian protection is a primary purpose of UN peacekeeping, yet there is little systematic evidence for whether peacekeeping prevents civilian deaths. We propose that UN peacekeeping can protect civilians if missions are adequately composed of military troops and police in large numbers. Using unique monthly data on the number and type of UN personnel contributed to peacekeeping operations, along with monthly data on civilian deaths from 1991 to 2008 in armed conflicts in Africa, we find that as the UN commits more military and police forces to a peacekeeping mission, fewer civilians are targeted with violence. The effect is substantial—the analyses show that, on average, deploying several thousand troops and several hundred police dramatically reduces civilian killings. We conclude that although the UN is often criticized for its failures, UN peacekeeping is an effective mechanism of civilian protection.