This chapter focuses on the threshold issue when international humanitarian law (IHL) applies to violence involving terrorism or terrorist groups, in the context of international or non-international armed conflicts. It discusses the particularly complex problem of ‘transnational’ violence and the geographical and temporal scope of hostilities. It then considers the legal consequences of the classification of conflicts, as regards targeting, detention, substantive criminal liabilities, and criminal trial procedure. Overall the challenge of terrorism has principally impelled a clarification of existing IHL norms but without generating terrorism-specific rules or refashioning IHL’s basic norms. Terrorists can be targeted for direct participation in hostilities; administratively detained where they are dangerous; and prosecuted for war crimes. Human rights law applies alongside the lex specialis of IHL to supplement its rules in certain areas, particularly as regards detention in non-international conflict. There is no need for any special status of ‘terrorist’ in IHL, which would only serve to diminish existing humanitarian protections. The chapter concludes with observations about the impact of international counter-terrorism law on the effectiveness of IHL, including its balance between military necessity and humanitarian protection.