Drawing on oral testimony, previously unseen personal papers, and newly released archival information, this book provides a comprehensive account of British and American intelligence on the Soviet nuclear weapons program from 1945-1958. The book charts new territory, revising traditional accounts of Anglo-American nuclear relations and intelligence cooperation. It reveals how intelligence was collected: the roles played by defectors, aerial reconnaissance, and how novel forms of espionage were perfected to penetrate the Soviet nuclear program. It documents what conclusions were drawn from this information, and assesses the resulting estimates. Throughout the book a central theme is the Anglo-American partnership, depicting how it developed and how legal restrictions could be circumvented by cunning and guile.