Following the September 11, 2001, attacks on the World Trade Center and the Pentagon, the United States established a strong and sophisticated counterterrorism enterprise to reduce the threat of large-scale terrorist attacks on the homeland. More than 20 years later, the terrorist threats are more ideologically and geographically diffuse. At the same time, the United States is confronting a diverse and dynamic range of other national security challenges, including strategic competition, cybersecurity threats, and climate change. To tackle evolving and emerging terrorist threats within the context of broader national security priorities, the United States inaugurated a new counterterrorism policy, shifting from a U.S.-led, military-centric approach to one that prioritizes diplomacy, partner capacity building, and prevention. Striking a new balance between military and civilian counterterrorism efforts recognizes the need to deploy the full range of counterterrorism (CT) tools and ensures a more sustainable whole-of-government and whole-of-society CT approach with allies and partners around the world. This book is a brief overview of the United States' ongoing counterterrorism efforts.