This Chapter investigates the principal challenges to the rule of law and human rights that have emerged during the United Kingdom’s criminal justice response to foreign fighters. The vast scope of the United Kingdom’s definition of terrorism in Terrorism Act 2000Terrorism Act 2000 section 1 ensures that the activities of foreign fighters are captured through the prism of counter-terrorism law but achieves this aim at the expense of the rule of law, in particular the principle of legalityPrinciple of legality. The Chapter explores the numerous legal avenues to prosecute foreign fighters through the existing legislative framework of the Terrorism Acts, analysing the strengths and weaknesses of UK law in addressing the foreign fighter phenomenon through the lens of counter-terrorism. In this context, it assesses impact of the UK definition of terrorism on the legal response to foreign fighters and its applicability to armed conflict, through an in-depth analysis of recent developments in the case law. Particular attention is given to the adoption of the ‘Designated Area Offence’, a controversial legal regime with flawed mens reaMens rea requirements as well as issues regarding international cooperation in the case of Kotey and El Sheikh which revolved around the risk of being subjected to the death penalty. Finally, the Chapter explores the international (human rights) law implications of the targeting of British foreign fighters through airstrikes by Coalition forces.