Numerous empirical studies have examined the role of third-party peacekeeping in reducing violence around the world. Their results reveal an extraordinary relationship between peacekeepers and peace, notwithstanding a number of well-known problems. This review article has three goals. The first is to summarize the results of past empirical research to move the debate beyond the question of whether peacekeeping works to the more pressing questions of how, when and why it works. The second is to reveal the limitations of the current quantitative research in order to identify areas in which scholars can make big, new contributions to the field. The final goal is to propose a new research agenda that is heavily evaluative – one that informs policy makers about the specific practices, mission compositions, and mandates that work, and identifies the local, regional, and international conditions that amplify or diminish peacekeeping's effectiveness. This type of research could help reduce the costs of peacekeeping operations, eliminate some of the negative consequences of interventions and save even more lives.