The participation of European foreign fighters in the recent armed conflict in Syria and Iraq poses a particularly intricate security conundrum in the field of counter-terrorism. This thesis assesses to what extent legal responses to foreign fighters at the international, European, and domestic level have been consistent with the rule of law and human rights. As the inherently transnational nature of the foreign fighter phenomenon has resulted in a labyrinth of legal obligations, the thesis scrutinises the complex interplay between legal norms at these three different levels. This assessment evaluates international and European legal instruments as well as domestic legal measures in the United Kingdom and the Netherlands against international rule of law and human rights standards. Throughout Europe’s legal response to foreign fighters four fundamental challenges have emerged which have prevented states and international organisations from resolving the foreign fighter conundrum in accordance with the rule of law and human rights. Firstly, the UN Security Council’s international legal regime on foreign fighters is premised on the flawed concept of the ‘foreign terrorist fighter’ yet fails to define terrorism. Overly capacious definitions of terrorism have proved to be inconsistent with the principle of legality and unsuitable to address the peculiarities of the modern-day foreign fighter phenomenon. Secondly, the thesis argues that attempts to resolve the foreign fighter conundrum through multilateral channels have by and large been inconsistent with the rule of law and human rights and have failed to add value to domestic responses. Thirdly, the foreign fighter conundrum has exacerbated the conflation between counter-terrorism law and international humanitarian law, to the detriment of the latter. Fourthly, the thesis analyses how legal responses to foreign fighters have sharpened the preventive turn that counter-terrorism law has taken in the aftermath of 9/11, at the expense of the rule of law and human rights.