In this article author analyzes the activities of international organizations in the area of preventing and combating international terrorism before and after 11 September 2001. Author notices key mechanism, tools, patterns and paradoxes of these activities. Text covers the activities of ten organizations: United Nations, European Union, North Atlantic Treaty Organization, Council of Europe, Organization of Security and Cooperation in Europe, League of Arab States, Organization of the Islamic Conference, African Union, Organization of American States, and Interpol. Author concludes that the evolution of activities of international organizations can be described in terms of radical change of perspectives on international terrorism among international community after terrorist attack of 11 September. What used to be phenomenon perceived as significant at times, but definitely not central or heavily strategic problem was suddenly elevated to the existential threat to international peace and security. One of the most significant aspects of this change has been growing interest of Security Council in international terrorism: during over half century before 9/11 it adopted only a few resolutions concerning this subject. Between 2001 and 2005 the number was almost twenty and Security Council created new institutions and mechanism such as Counter-Terrorism Committee (CTC). Regional organizations after 9/11 also started to treat terrorism as one of their highest priorities if not THE priority. Their activities included among others new normative instruments as Conventions, Protocols, Plans of Actions and new multilateral bodies and institutions. Still, there are many unresolved issues making international cooperation harder than is should be. Perhaps the most important of these issues is inability of international community to create and adopt global comprehensive counterterrorism convention. There are still fundamental differences between states considering the definition of terrorism, rooted in their ideologies, historical experiences, interests, prejudices and lack of trust.