How optimistic or pessimistic should we be about the future spread of nuclear weapons, and to what levers do policymakers have access to reduce the likelihood of proliferation? Concerns about accelerating proliferation are rising again, as they have periodically during the sixty-plus years of the nuclear age. This time, Iran’s apparent pursuit of at least a nuclear weapons option is widely expected to catalyze regional proliferation, and there is also concern about further proliferation in response to North Korea’s acquisition of at least rudimentary nuclear weapons. Both theoretical and empirical scholarship can shed light on the conditions under which states are more or less likely to proliferate and the levers policymakers can use to make proliferation less likely. Students will gain knowledge of the development and current state of proliferation theory, survey past works on forecasting nuclear proliferation, analyze prior nuclear proliferation decisions and trigger events, and will be expected to combine theory with comparative studies in conducting their original research.