This study reflects the most comprehensive, publicly available accounting of Americans who traveled to join jihadist groups in Syria and Iraq since 2011. It identifies 64 travelers, the largest available sample to date. These individuals, and their stories, were uncovered during a multi-year investigation. Authors interviewed law enforcement officials, prosecutors, and defense attorneys, and attended relevant court proceedings. Additionally, they reviewed thousands of pages of legal documents, filing information requests and federal court motions to unseal records where necessary. Finally, the authors conducted several interviews with American travelers who returned from the territories held by the Islamic State (IS).