What is nuclear secrecy? This fascinating book shows that the answer is more interesting and consequential than you might think. Drawing on extraordinarily rich sources – archival documents, photographs, and technical drawings – Alex Wellerstein traces the changing form of nuclear secrecy in the United States. His history is rich in detail, but also in tracing the evolving shape and consequences of such secrecy. Initially developed through self-regulated silence by scientists, the US secrecy regime became increasingly expansive after the Second World War. US priorities have evolved from advancing in the nuclear competition during the Cold War toward a reckoning with the darker secrets, such as human radiation experiments, in the early 1990s. The shape, form, and purpose of nuclear secrecy in the United States has been continuously contested, from within and without the nuclear establishment, and changed. Nonetheless, ‘restricted data’, the specific legal category of nuclear secrets that the US created to protect its emerging nuclear monopoly, and subsequently to slow the spread of nuclear weapons to other states, has become a totemic feature of US statecraft. The scope of the challenge, where scientists are profoundly skeptical about keeping science and technology secret and commercial actors compete with government agencies in reshaping the balance between restriction and dissemination, is in many ways astounding. Wellerstein offers thoughtful reflections on how much scholars can and should know about secrecy, particularly when working exclusively with declassified sources. Restricted Data is an explicitly US-specific history of nuclear secrecy, but one that reflects how other states and actors have shaped what can and should be restricted.