Abstract : Despite the heightened interest placed in stemming the proliferation of nuclear weapons and increased controversy about the effectiveness of the Nuclear Non-Proliferation Treaty (the NPT) regime, surprisingly little systematic scholarship has investigated the NPT: how does it function? Why do countries join it or hesitate to do so? Is the NPT a paper tiger, as critics claim, or the foundation of a successful anti-proliferation strategy? To answer this question, we need to understand why countries sign the NPT in the first place. If the NPT is a paper tiger that may even aid countries in advancing their nuclear technology, as critics allege, why did so many countries hesitate to sign it? On the other hand, if the NPT poses real restrictions, as advocates claim, then why do countries that could potentially benefit from a nuclear deterrent nonetheless sign away their right to make the bomb? Understanding who signs the treaty, why, and when is an important step towards a better understanding of how the NPT functions. We develop a simple cost-benefit framework to understand the pros and cons of joining the NPT, emphasizing economic, security, and political factors. We evaluate our framework with statistical models, covering 185 countries from 1968 to 2000, of decisions to join the NPT. Results suggest that eagerness to join the NPT is strongly associated with level of economic development, high energy needs, external threat environment, lack of great power security guarantees, democratic political arrangements, and political volatility.