The September 11, 2001, terrorist attacks on the United States and the subsequent revelation of Al Qaeda cells in Europe gave new momentum to European Union (EU) initiatives to combat terrorism and improve police, judicial, and intelligence cooperation among its member states. Other deadly incidents in Europe, such as the Madrid and London bombings in 2004 and 2005, respectively, injected further urgency into strengthening EU counterterrorism capabilities. Among other steps, the EU has established a common definition of terrorism and a common list of terrorist groups, an EU arrest warrant, enhanced tools to stem terrorist financing, and new measures to strengthen external EU border controls and improve transport security. Over the years, the EU has also encouraged member states to devote resources to countering radicalization and terrorist recruitment; such efforts have received renewed attention in light of concerns about the threats posed by European fighters returning from the conflicts in Syria and Iraq, highlighted most recently by the November 13, 2015, attacks in Paris, France. Promoting law enforcement and intelligence cooperation with the United States has been another top EU priority since 2001. Washington has largely welcomed enhanced counterterrorism cooperation with the EU. Since 9/11, contacts between U.S. and EU officials on police, judicial, and border control policy matters have increased substantially. A number of U.S.-EU agreements have been reached; these include information-sharing arrangements between the United States and EU police and judicial bodies, U.S.-EU treaties on extradition and mutual legal assistance, and accords on container security and airline passenger data. In addition, the United States and the EU have been working together to curb terrorist financing, strengthen transport security, and address the foreign fighter phenomenon. Nevertheless, some challenges persist in fostering closer U.S.-EU cooperation in these fields. Among the most prominent and long-standing are data privacy and data protection issues. The negotiation of several U.S.-EU information-sharing agreements, from those related to tracking terrorist financial data to sharing airline passenger information, has been complicated by EU concerns about whether the United States could guarantee a sufficient level of protection for European citizens' personal data. EU worries about U.S. data protection safeguards and practices were further heightened by the unauthorized disclosures of U.S. National Security Agency (NSA) surveillance programs in mid-2013 and subsequent allegations of U.S. collection activities in Europe. Other issues that have led to periodic tensions include detainee policies, differences in the U.S. and EU terrorist designation lists, and balancing measures to improve border controls and border security with the need to facilitate legitimate transatlantic travel and commerce. Congressional decisions related to data privacy, intelligence-gathering, border controls, visa policy, and transport security may affect how future U.S.-EU counterterrorism cooperation evolves.