The authors of this article examine the role of the tacit knowledge embodied in human intelligence in the development and spread of nuclear weapons. They contrast explicit knowledge that can be formulated and stored as words and symbols with tacit knowledge, which is intuitive and thus cannot be formulated explicitly. They focus on the loss of tacit knowledge through the cessation of nuclear weapons design due to disarmament efforts and suggest that renewed design efforts would have the characteristics of novel invention rather than just copying earlier designs. Numerous examples from the nuclear weapons programs of the United States, the Soviet Union, Britain, France, Israel, India, South Africa, and Pakistan are cited in support of the authors’ theory. They conducted interviews with numerous current and retired staff members of Los Alamos and Lawrence Livermore National Laboratories in support of their work.