Contemporary terrorism is a product of globalization, a network which spreads across several countries, whose ambitions are geopolitical rather than simply national, which uses the media and is prepared to cause greater damage than the traditional forms of terrorism. The Members of the International Community must therefore ensure that all measures taken to combat terrorism are compatible with the obligations they have in virtue of International Law, and particularly the principles of human rights, rights of refugees, and International Humanitarian Law. However, it is not entirely clear whether International Law offers legitimate, valid and effective responses to terrorist threats. This new terrorism is upsetting some essential categories of International Law, particularly concerning the law of legitimate self- defense and demands a narrower cooperation between States in policy and judicial matters as well as new initiatives in other specific matters. This book tries to offer a realistic and practical analysis about how to update the international structures and institutions to this new social reality.