Certain regions of the world experience more conflict than others. Previous analyses have shown that a civil war in one country significantly increases the likelihood that neighboring states will experience conflict. This finding, however, still remains largely unexplained. We argue that population movements are an important mechanism by which conflict spreads across regions. Refugee flows are not only the consequence of political turmoil—the presence of refugees and displaced populations can also increase the risk of subsequent conflict in host and origin countries. Refugees expand rebel social networks and constitute a negative externality of civil war. Although the vast majority of refugees never directly engage in violence, refugee flows may facilitate the transnational spread of arms, combatants, and ideologies conducive to conflict; they alter the ethnic composition of the state; and they can exacerbate economic competition. We conduct an empirical analysis of the link between refugees and civil conflict since the mid-twentieth century, and we find that the presence of refugees from neighboring countries leads to an increased probability of violence, suggesting that refugees are one important source of conflict diffusion.We would like to thank the participants in the “Resources, Governance Structures, and Civil War” Workshop at the European Consortium for Political Research in Uppsala, Sweden, 13–18 April 2004, for early feedback on previous versions of this article. We would also like to thank Anis Bajrektarevic, Lars-Erik Cederman, David Cunningham, Kristian Berg Harpviken, Béla Hovy, Sarah Lischer, Monty Marshall, Erik Melander, Will H. Moore, Magnus Öberg, and Michael Ward for providing us with data and helpful comments, as well as Jan Ketil Rød for permission to reproduce the map from the program ViewConflicts in Figure 1. Finally, we are grateful for the comments and suggestions of the editors of International Organization and the anonymous reviewers. This research was supported by a grant from the National Science Foundation (SES-0351670).