This Chapter examines the far-reaching international legal obligations to address the foreign fighter phenomenon that the UN Security Council imposed on states by adopting Resolution 2178 and Resolution 2396, adding a new layer to the UN counter-terrorism architecture as developed in the aftermath of 9/11. The Chapter argues that the Resolution’s crucial flaw is the introduction of the hybrid legal concept of the ‘foreign terrorist fighter’ without providing explicit limits and safeguards to its application. States are required to prevent and suppress the recruitingRecruiting, organising, transporting or equipping of ‘foreign terrorist fighters’; to prosecute and to penalise ‘foreign terrorist fighters’; and to prevent the movement of ‘foreign terrorist fighters’. The sheer width of the international legal obligations aimed at ‘foreign terrorist fighters’, combined with the fact that these individuals are an entirely new category of legal actors, amount to an international legal regime on foreign fighters in its own right that has had major impact on supranational an domestic responses. The Chapter concludes that the Security Council’s decision to introduce the ill-defined legal concept of the ‘foreign terrorist fighter’ into international law creates an unnecessary conflation of legal categories that is inconsistent with legal certainty. Finally, the Chapter demonstrates that the Security Council has acted ultra viresUltra vires by imposing general and abstract obligationsGeneral and abstract obligations on states to address ‘foreign terrorist fighters’ rather than obligations restricted to the specific armed conflict in Syria and Iraq, making the regime inconsistent with key principles of international institutional law.