In this article I argue that it is much harder to deter and compel non-nuclear states and terrorist groups through the threat of use of nuclear weapons than proponents of nuclear use contend. A counter-proliferation strategy relying on nuclear threat and preventive war has serious limitations and may well be a source of nuclear proliferation rather than non-proliferation. While the fear of a retaliatory attack constrains a nuclear state from using its nuclear weapons against another nuclear state, a nuclear state may not be able to mount and execute a nuclear retaliatory strike against a non-nuclear state or a non-state actor for reasons beyond military calculations. The nuclear state could be restrained by self-imposed reputational concerns arising from moral, legal, and other normative considerations. This form of restraint can be aptly termed ''self-deterrence.'' This article first elaborates the concept of self-deterrence and then explores the core reasons for its prevalence. There may be multiple reasons for self-deterrence, including domestic politics, bureaucratic politics, and leaders' psychology, especially in terms of risk aversion. However, in light of the historical record-especially instances from US nuclear history-reputational considerations appear to be crucial in explaining selfdeterrence. These reputational considerations derive largely from three sources: the tradition of non-use of nuclear weapons, moral restraints, and legal principles regarding the use of nuclear weapons. I conclude by arguing that deterrence theory and policy need to take into account this aspect of self-deterrence along with cultural, psychological, and domestic-level constraints that have been presented as challenges to that theory's premises and applicability.