The Legacy of 9/11 is a retrospective about how policing, intelligence, and counter-terrorism have changed in the more than twenty years since the September 11, 2001 terrorist attacks. Bringing together scholars and practitioners, the book takes an interdisciplinary approach with fields including history, international relations, intelligence studies, law, and political science. It highlights how some challenges in policing, intelligence, and counter-terrorism brought about by the attacks have been resolved, how some persist and how others have been transformed. The chapters explore state and non-state actors’ actions, reactions, and overreactions that shape contemporary aspects of policing, intelligence, and terrorism. In all three worlds, intelligence, policing, and counter-terrorism, the 9/11 attacks changed how the threat of terrorism is perceived, approached, and effectively countered by learning from the mistakes that led to the success of the attacks and initiating a process on the national and international levels of integrating security structures and implementing changes that have made 9/11 the last large scale terrorist strike on U.S. soil. To illustrate these accomplishments and to highlight future challenges, the volume examines the inextricably connected elements of policing and intelligence in counter-terrorism as well as how counter-terrorism practitioners and jihadists were transformed by one day of attacks, more than twenty years ago. The chapters in this book were originally published as a special issue of Journal of Policing, Intelligence and Counter Terrorism.