This volume of essays, drawn from a Hague Academy of International Law' Research Centre session, tackles one of the major challenges with regard to contemporary international law. Terrorism hits at the very nature of interstate relationships since it is defined by the action of non-State actors against States, and since it gives rise in return to reactions which go through multiple channels some of which were previously unused. The sheer mobility of terrorist activity, its capacity to provoke asymmetrical confrontations, creates yet another difficulty. This collection attempts to deal with some of the complicated questions that arise from the fact that terrorism challenges and puts into question peaceful international relations in all their different dimensions.