While previous research shows that civil wars can spread to neighboring states, we do not know why certain neighbors are more at risk than others. To address this research gap, this dissertation proposes a contagion process approach that can identify the most likely targets of contagion effects from an ongoing conflict. Using data with global coverage, theoretical expectations about why and where civil wars would have contagion effects, are examined in a series of statistical analyses. Paper I argues and empirically supports that a country is more susceptible to contagion effects when it is characterized by ethnic polarization, where few ethnic groups form a delicate balance. Paper II argues and provides evidence that the involvement in conflict by an ethnic group in one country increases the likelihood of ethnic conflict erupting in a neighboring country that shares the same ethnic group. Paper III suggests and finds support that the arrival and long-term hosting of refugees from states in civil conflict make host states more likely to experience civil conflict. Paper IV examines the common notion that the granting of autonomy or independence to separatist groups may spur other ethnic groups to violently pursue similar demands, starting off a domino effect. Using new global data on such territorial concessions, the analysis does not support this version of the “domino theory,” which is popular among policy-makers. In sum, this dissertation contributes by demonstrating the usefulness of the contagion process approach. It offers a more comprehensive view of contagion among neighbors, and as such is able to specify arguments and intuitions in previous research.