A nuclear detonation in the United States is one of the most catastrophic incidents imaginable. While the United States Government is working domestically and with international partners to ensure this scenario never occurs, failing to plan for managing the consequences of such an event would be irresponsible. Should a nuclear detonation occur, a crucial task for federal, state, local, tribal and territorial authorities and private-sector organisations will be communicating clear and consistent messages to the public. All levels of government have responsibility for co-ordinating and communicating information regarding the incident to the public immediately after a nuclear detonation. State, local and tribal authorities retain the primary responsibility for responding to large-scale incidents, such as a nuclear detonation. Effectively communicating health and safety instructions to the population will be a critical factor in building trust, comforting the nation, saving lives and minimising injury. This book was developed as a resource for emergency responders and federal, state, and local officials communicating with the public and the media during the immediate aftermath of a nuclear detonation in the United States. An interagency group of communications and radiation technical experts developed the messages in this book, which include key messages for the impacted community and the nation, and anticipated questions and answers for distribution to the public in the immediate aftermath of a nuclear detonation.