This volume provides the most comprehensive and up-to-date collection of theoretical perspectives regarding the sources of and propensity for nuclear proliferation. The authors probe the broader questions of why states pursue or abstain from nuclear weapons, as well as finer methodological issues involving concept definition and development, hypothesis testing, and generalization of findings. They draw upon both the extensive body of qualitative analysis and the inchoate but important work of a quantitative nature. Although the chapters do not all focus specifically on the relationship between one state's nuclear behavior and that of another, collectively the essays provide a better understanding of the limits of reactive proliferation as well as the circumstances under which weapons diffusion is most likely to occur. They also offer compelling arguments about what must be done in order to improve proliferation prognoses and propose new conceptual approaches toward that end.