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.