Scholars often point to pressure from the United States as a key factor in driving membership in the nuclear nonproliferation regime, but this explanation has trouble explaining the changes we see in patterns of membership over time. This chapter shows that variation in the perceived effectiveness of the regime – as indicated by overall membership, the strength of verification measures, the effectiveness of enforcement, and a history of cooperation – better explains why states join. Member states are reluctant to forgo nuclear weapons without assurances that others will comply as well, and signals of regime effectiveness reassure states that their commitments will be reciprocated. This argument runs counter to the conventional wisdom among international organizations scholars, that there is a “depth versus breadth” tradeoff in institutional design. Drawing from archival documents, this chapter discusses the illustrative cases of the NPT ratification decision in Australia and Switzerland. It then tests its theory using data on state membership in the NPT.