Previous quantitative work on the causes of foreign intervention in civil conflict has focused on material determinants of intervention. Studies that do investigate affective factors such as ethnic affinities and their association with interstate conflict and intervention, tend to under-specify the linkages between structural characteristics of ethnic groups and the likelihood and partiality of interventions. This thesis attempts to better specify the conditions under which ethnic affinities cause states to intervene in civil conflicts abroad, as well as the conditions that determine whether interventions favour government or opposition in a civil war. Hypotheses on the probability of intervention are based on whether transnational ethnic affinities exist between a potential intervener and a country in civil conflict, and the extent to which the ethnic group in power in the intervener is domestically predominant. The choice to intervene is modelled formally, using expected utility equations and contest success functions. Hypotheses on the side of intervention - whether interventions favour government or opposition - are derived from an interstate dyad typology based on whether transnational ethnic affinities run between two ethnic groups in power, two non-governmental ethnic groups, or between one non-governmental ethnic group and one ethnic group in power. Analysis of cross-sectional data on civil conflicts in North Africa and Eurasia suggests that transnational ethnic affinities and domestic ethnic predominance have robust and sizeable positive effects on the likelihood of intervention. The findings also support the effort to predict the side of interventions with reference to configurations of transnational ethnic groups and power.