In this introductory essay, we aim first to demonstrate why the question of which states will develop nuclear power in the future matters for global security. To do so, we briefly discuss the connections between nuclear power, nuclear proliferation, and terrorism risks; we present data contrasting existing nuclear-power states with potential new entrants with respect to factors influencing those risks. Second, we introduce major themes addressed by the authors in both volumes, and explain why the expansion of nuclear power, the future of nuclear weapons disarmament, and the future of the NPT and related parts of the nuclear control regime are so intertwined. Finally, we conclude with some observations about what is new and what is not new about current global nuclear challenges. The American Academy of Arts and Sciences has published three important special issues of Dædalus on nuclear weapons issues in the past-in 1960, 1975, and 1991-and reflecting on the differences between the concerns and solutions discussed in those three issues and the nuclear challenges we face today is both inspiring and sobering.