The changing rules on the use of force in international law considers the main legal issues concerning the use of force by international organisations and states. It assesses the achievements and failures of the United Nations' collective security system, and discusses the prospects ahead. It also deals with the use of force by states in self-defence and on other legal grounds. The book discusses to what extent the rules on the use of force have evolved since the end of the Cold War in order to meet the needs of the international community. It focuses in particular on the military operations directed against terrorism and weapons of mass destruction. The research is developed from the standpoint of the sources of international law. It rejects a static vision of the rules on the use of force, including those enshrined in the UN Charter. Rather, it highlights the interaction between conventional and customary international law and the exposure of both sources to state practice.