Nuclear weapons proliferation is a topic of intense interest and concern among both academics and policy makers. Diverse opinions exist about the determinants of proliferation and the policy options to alter proliferation incentives. We evaluate a variety of explanations in two stages of nuclear proliferation, the presence of nuclear weapons production programs and the actual possession of nuclear weapons. We examine proliferation quantitatively, using data collected by the authors on national latent nuclear weapons production capability and several other variables, while controlling for the conditionality of nuclear weapons possession based on the presence of a nuclear weapons program. We find that security concerns and technological capabilities are important determinants of whether states form nuclear weapons programs, while security concerns, economic capabilities, and domestic politics help to explain the possession of nuclear weapons. Signatories to the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) are less likely to initiate nuclear weapons programs, but the NPT has not deterred proliferation at the system level.