The two books under review, The tradition of non-use of nuclear weapons, by T. V. Paul and Deterrence: from Cold War to long war. Lessons from six decades of RAND research, by Austin Long, highlight the continued interest in the theory and practice of nuclear deterrence. Long traces the RAND Corporation's research on the subject, exploring the role that nuclear deterrence has played as a strategy of the Cold War. The author goes on to argue for the relevance of nuclear deterrence to the future strategic environment, considering threats from peer-competitors to non-state actors. By contrast Paul considers the rise and persistence of a tradition, or informal social norm, of non-use which has encouraged self-deterrence. Employing a series of examples, Paul argues that this tradition best explains why, since 1945, nuclear states have not used nuclear weapons against non-nuclear opponents. Taken together, these books encourage further consideration of the relationship between nuclear deterrence and the tradition of non-use. Indeed, it is difficult to see how the two practices can successfully coexist if non-nuclear states have, as Paul suggests, already begun to exploit the existence of a tradition of non-use. Such deterrence failures, real or perceived, have profound implications for relationships between nuclear and non-nuclear states.