In this chapter, Casper Sylvest explores the role of nuclear weapons in intellectual history during the early decades of the Cold War, predominantly in the US and Europe. The chapter opens with a discussion of the role of nuclear weapons technology in transforming both scientific knowledge about the planet and the landscape of intellectual debate. Sylvest then turns to the conceptions of this technology among policymakers, military figures, scientists and public intellectuals. Four sites of contestation are singled out: the question of morality, the question of use, the question of stability and a more amorphous set of questions associated with the human condition in the nuclear age. In conclusion, Sylvest reflects on the nature of nuclear weapons and our historical understanding of them.