European citizens commonly described as ‘foreign fighters’ who have been involved in the armed conflict in Syria and Iraq and the rise and fall of the ‘Islamic State’ pose an intricate security challenge for European governments and law enforcement officials. This Chapter first outlines what European foreign fighter conundrum entails. The Chapter then discusses the significance of the main research question of the book: To what extent has the European legal response to foreign fighters been consistent with the rule of law and human rights? The inherently transnational nature of the foreign fighter phenomenon has created a messy labyrinth of legal obligations at the international, supranational, and domestic level. This book scrutinises the complex interplay between legal norms at these different levels. The first part of the book zooms in on international and supranational responses to foreign fighters in Syria and Iraq. The book proceeds with a comparative legal analysis of the implementation of international and European legal obligations in the domestic legal frameworks of the United Kingdom and the Netherlands, which focus on criminal justice responses. This is followed by a selected thematic dimension: the deprivation of citizenship. This Chapter defines key concepts used throughout the book such as ‘foreign fighters’ and the ‘rule of law’ and provides an overview of the book’s methodology and sources.