Why are some groups of peacekeepers more effective at managing civil war violence than others? Existing studies of operational effectiveness have focused on the quantity, profession, and geographic location of deployed personnel and have suggested that each of these is independently important. However, the relationship, if any, between the quality of UN peacekeepers and their ability to manage on-going violence remains critically understudied. To address this gap, this study begins by developing a novel definition of "peacekeeper quality" that consists of two key factors: professional capabilities and a willingness to act. It then evaluates the extent to which variation in peacekeeper quality contributes to operational effectiveness, at both the state-level and the local-level. I argue that higher quality peacekeepers are more effective than lower quality peacekeepers because they have an easier time coercing local conflict actors. To test this argument, I use an explanatory sequential methodology - a form of mixed method research that uses qualitative data to help interpret the results of a primarily quantitative study. It starts with a cross-national analysis of all intrastate conflicts in sub-Saharan Africa from 1991-2017 to determine if variation in average operational quality affects the severity of violence at the state-level. It then proceeds to a geographically disaggregated analysis of three peacekeeping operations located in sub-Saharan Africa from 2010-2017 to determine if variation in the average quality of individual groups of peacekeepers affects the severity of violence at the local-level. Finally, it presents a process tracing case study of MINUSCA's intervention in CAR from 2014-2018 that examines both overall trends and discrete conflict episodes. In short, this dissertation shows how and why peacekeeper quality matters. Specifically, it shows that the UN's highest quality peacekeepers are its most effective, that the quantity of peacekeepers continues to affect violence when controlling for quality and that peacekeepers are better at protecting civilians from harm than they are at managing battlefield violence. As a result, future attempts to evaluate the relationship between peacekeepers and violence, or to improve the UN's operational effectiveness, should also account for the role played by peacekeeper quality.