Since the early 1990s, the U.S. Nuclear Weapons Program has evolved significantly as a result of unilateral and bilateral arms reductions and the end of underground nuclear testing in the United States, successive editions of these books have been revised and restructured to reflect these changes. This book is intended to be an unofficial reference that explains the history and development of the U.S. Nuclear Weapons Program as well as the current activities associated with sustaining the U.S. nuclear deterrent. It is designed to be useful, but it is neither authoritative nor directive. Please refer to the applicable statute, regulation, Department of Defense Direction/Instruction, or Department of Energy Order for definitive guidance in all areas related to the U.S. Nuclear Weapons Program. The content of Nuclear Matters: A Practical Guide is the sole responsibility of the Office of the Deputy Assistant to the Secretary of Defense for Nuclear Matters.