For more than fifty years, the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) and the wider nuclear nonproliferation regime have worked to prevent the spread of nuclear weapons. Analysts and pundits have often viewed the regime with skepticism, repeatedly warning that it is on the brink of collapse, and the NPT lacks many of the characteristics usually seen in effective international institutions. Nevertheless, the treaty continues to enjoy near-universal membership and high levels of compliance. This is the first book to explain why the nonproliferation regime has been so successful, bringing to bear declassified documents, new data on regime membership and weapons pursuit, and a variety of analytic approaches. It offers important new insights for scholars of nuclear proliferation and international security institutions, and for policymakers seeking to strengthen the nonproliferation regime and tighten international constraints on the spread of nuclear weapons.