This article examines how foreign fighters were recruited and mobilized for Islamic State in 2012-2014. Institutional and individual approaches to this phenomenon form the basis of understanding the mechanisms used for the mobilization and recruitment of foreign fighters. The former refers to a terrorist institution that plays a key role in the recruitment of individuals (top-down/institutional), and the latter refers to the self-radicalization process that foreign fighters undergo (bottom-up/ individual). In particular, the research focuses on an analysis of Islamic State and the recruitment/mobilization of sixteen foreign fighters from Tunisia, Saudi Arabia, France, and the United Kingdom. The analysis shows that both top-down and bottom-up concepts are important but that the extent to which each is used depends on the profiles of the country in question. The study concludes by demonstrating the comparative value of top-down and bottom-up approaches in terms of understanding contemporary terrorist recruitment and providing policy recommendations.