The United Nations (UN) is the most significant organization in the fight against terrorism. This paper details the legal basis for the UN's counter-terrorism (CT) role, quoting both the UN Charter and relevant resolutions. Since 2005 the UN has a Counter-Terrorism Implementation Task Force (CTITF), heading up coordination work with numerous international bodies. The following year saw the adoption of the UN Global Counter-Terrorism Strategy. Also in the wake of 9/11, but also of the Madrid and London bombings, the European Union (EU) has taken an active role. The EU Action Plan to Fight against Terrorism dates from 21 September 2001. In 2005 The EU committed itself to a CT Strategy, covering four strands of work: prevention, protection, pursuit, and response. The third organization examined in this chapter is the North Atlantic Treaty Organization (NATO), which endorsed a Military Concept for Defence against Terrorism at the 2002 Prague Summit. Here again there are four roles: anti-terrorism, consequence management, CT, and military cooperation. The Alliance has recently developed a NATO Response Force, with over 20,000 troops. There follow details of NATO operations relating to CT. In conclusion, cooperation among UN, EU, NATO and others should be enhanced, since it is necessary to take an integrated approach to CT.