"Taking stock of the fact that the participation of foreigners in armed conflicts has attracted much attention over the past years and has been increasingly considered a threat to international peace and security, the thesis aims at addressing the legal aspects surrounding the situation of the category of individuals that has become known as "foreign fighters" or "foreign terrorist fighters" and the persons affiliated to them ("affiliated persons"). Using the events in Syria and Iraq following the Syrian civil war as a case-study, the thesis will approach the question from a practical perspective, identifying and providing a thorough legal analysis of some of the main challenges that have arisen in this context. The thesis argues that, while foreign fighters and foreign terrorist fighters do not constitute a new legal category per se, and therefore are not subject to a distinct set of norms, (1) it appears from practice that they do face a unique and consistent set of issues of international law, and (2) the rights and duties of states and of the individuals concerned are interpreted in a specific way as a result of their categorization as foreign fighters and/or affiliated persons."