This chapter examines the application of international law to ISIS. Four key areas of international law (IL) are examined: international humanitarian law (IHL), international criminal law (ICL), international human rights law (IHRL), and customary international law (CIL). Next, issues associated with applying these laws to ISIS are discussed. Both legal and practical difficulties are considered, particularly definitional, jurisdictional and prosecutorial issues. The underlying framework offered by IL is also found to be insufficient to address the challenges presented by the emergence of globalized, religious terrorism. Therefore, international law has to evolve to improve the international legal accountability of both ISIS and states engaged in military actions against such organisations. This chapter concludes by evaluating methods for improving IL, including the concept of an international counter-terrorism court.