In the fight against the scourge of terrorism, a large number of international organizations are engaged in enhancing cooperation among States. The European Union represents one of the best examples of a regional organization fostering the coordination of efforts made by its member states in a spirit of solidarity. The terrorist attacks in Madrid and London have proved that Europe is vulnerable to terrorism. Consequently, the EU has developed in recent years a common counterterrorism strategy for its member states based on four key activities: prevent, protect, pursue and respond. Specialized EU agencies have been set up (Eurojust) and others redefined (Europol) to make the exchange of intelligence and interstate judicial cooperation more effective. Legal tools such as a common European definition of terrorism and the European Arrest Warrant have been introduced to strengthen the counterterrorism cooperation in a harmonized legal framework. Also, to reduce the vulnerability to a bioterrorist attack, a strategy for preparedness and response to such a threat has been adopted. Although national security remains a responsibility of member states, there is a fundamental role for the EU to play in developing a common response, especially in the light of the new provisions of the Lisbon Treaty that seem to pave the way for a more coherent EU institutional framework and for a more efficient decision-making process.