The rising number of U.S. and European citizens traveling to fight with rebel and terrorist groups in Syria and Iraq has emerged as a growing concern for U.S. and European leaders, including Members of Congress. Several deadly terrorist attacks in Europe over the past year -- including the killing of 17 people in Paris in January 2015 -- have heightened the perception that these individuals could pose a serious security threat. Increasingly, terrorist suspects in Europe appear to have spent time with groups fighting in the Middle East, especially with the Islamic State organization (also known as ISIL or ISIS). Others, like the gunman who murdered two individuals in Copenhagen in February 2015, seem to have been inspired by Islamist extremist propaganda. U.S. intelligence suggests that more than 20,000 foreign fighters have traveled to the Syria-Iraq region, including at least 3,400 Westerners, since 2011. The vast majority of Western fighters are thought to be from Europe, although roughly 150 Americans have traveled or attempted to travel to Syria. U.S. authorities estimate that a handful of Americans have died in the conflict; they also assert that military operations against the Islamic State group since August 2014 have killed thousands of fighters, including an unknown number of foreigners. European governments have employed a mix of security measures and prevention efforts to address the potential foreign fighter threat. This book discusses U.S. and European assessments of and responses to the foreign fighter phenomenon. It focuses on government policies primarily in Western European countries and analyzes EU measures to counter the foreign fighter threat given the EUs largely open internal borders and that 23 EU member states belong to the U.S. Visa Waiver Program. It also briefly evaluates foreign fighter outflows and responses in the Western Balkans and Russia; discusses U.S.-European cooperation, primarily in the law enforcement and intelligence areas, and addresses issues of particular concern for Congress, such as the VWP; examines the evolution of U.S.-EU counterterrorism cooperation and the ongoing challenges that may be of interest in the 113th Congress; and concludes with statements from several speakers of three different hearings related to foreign fighters.