The dissertation consists of three essays on the foreign militants who joined the Islamic State of Iraq and Syria (ISIS). The first essay examines the within-group role preference of ISIS soldiers. I look at the characteristics of the Islamic's States members who volunteer to be a suicide attacker instead of a fighter. I find that men who are less educated and have a basic knowledge of Islam are more likely to prefer to serve as a suicide attacker than a fighter. The results also show that men with previous combat experience and those are give large donations to ISIS are less likely to want to be a suicide attacker. The second essay takes a more typical approach to the study of terrorist participants and compares the demographic information of the ISIS foreign fighters to non-combatants from the same country in the Middle East. Consistent with past work on terrorist operatives, a university education is the strongest correlate of joining the Islamic State. In addition, an individual that holds an unskilled job but has a college education has a positive association with joining ISIS. This provides support in favor of the relationship between relative deprivation and extremism. Finally, this essay replicates well-known empirical findings on terrorist participants, such as age and marital status, using a new dataset and application. The third essay aggregates the individual ISIS foreign fighter observations up to the ethnic group level. I consider conventional explanations of ethnic conflict and determine whether they can extend to participation in ethno-transnational conflict. I find that income inequality is the strongest predictor of participation in foreign fighting. By contrast, political grievances have no association with transnational ethnic conflict. Overall, the results show that socio-economic grievances motivate foreign fighting.