International terrorism has been recognised as a global security threat. Prevention has become a critical challenge in the efforts to counter international terrorism, also affecting the legal responses to the scourge. Terms implying a more continuous state of affairs, such as 'terrorist networks' or 'terrorist infrastructure', have entered the legal discourse. New pro-active anti-terrorist criminalisations target different kinds of material support to terrorism. In the area of financial sanctions, a new form of international quasi-criminal accountability has been created. This book offers several perspectives from which to study the expansion and diversification of international legal responses to the terrorist threat during the past decade. The main focus is on the question of the broadening of international responsibility for terrorist acts and terrorism-related activities. The conclusions confirm that indirect contributions to terrorism are increasingly being outlawed. Preparatory criminal acts are seen as legal equivalents to completed crimes and, in terms of responsibility, a principle of non-differentiation applies. This development has been compared with recent developments in other areas of international criminal law, in particular the law of the core crimes (international criminal law sensu stricto) and its doctrines of extended responsibility. The study also covers several other fields of international law: international humanitarian law, the law of state responsibility and the UN Charter law on the use of force.