The NPT was met with skepticism when the treaty first went into force, and 50 years later analysts are still predicting its imminent demise. This chapter highlights the central puzzle of the book: why does the nuclear nonproliferation regime – which most expected to have limited effectiveness – appear to have been so successful? Tracing the history of the regime, it draws from declassified documents and diplomatic records to examine how events have shaped perceptions of the regime’s effectiveness. It describes the parallel expectations of international organizations and international security theory and contrasts the widespread pessimism about the regime with evidence of its success.