Fears of rogue states, withdrawal of cold war-era security guarantees, a falling technological threshold, and availability to terrorist organizations ensure that nuclear weapons proliferation remains a central security issue and that developing an adequate theory of proliferation ranks high on the agenda. A data set on nuclear proliferation is constructed that identifies three different stages on the path to the weaponization of nuclear weapons technology. Hazard models and multinomial logit are used to test theories of nuclear proliferation. Results suggest that nuclear weapons proliferation is strongly associated with the level of economic development, the external threat environment, lack of great-power security guarantees, and a low level of integration in the world economy.Sincetheadventoftheatomicage,nuclearweapons proliferation has been one of the major security issues facing the world. After the end of the cold war, concerns about proliferation have grown rather than subsided: the withdrawal of superpower security guarantees has created incentives for smaller powers to acquire nuclear weapons, a handful of "rogue" states have sought nuclear arms, Pakistan and India have joined the ranks of overt nuclear powers, the technological threshold necessary to develop atomic weapons is in reach of ever more nations, and the possibility of new nuclear powers selling weapons to terrorist organizations has focused concerns. Controversy rages around the world over U.S. plans to build a national missile defense (NMD) system to fend off emerging nuclear threats, and scholars debate whether NMD will fan the fires of proliferation or reduce the incentive for more states to acquire nuclear 859 AUTHORS' NOTE: Thanks to