This chapter explores why some states have chosen to build the bomb while others have not, the debate over the impact of nuclear proliferation and the significance of nuclear ages, before introducing the concept of nuclear latency. The first section looks at different models used to explain nuclear acquisition and retention and addresses the question of why some states have either given up nuclear bomb programmes or chosen not to build nuclear weapons at all. The chapter then looks at the challenges posed by the vertical and horizontal proliferation of nuclear weapons over the last seven and a half decades, and at how the number of weapons and number of actors has fluctuated over this time-period. Next, it explains the influence of viewing nuclear history as comprising two distinct “nuclear ages”, before looking at the proliferation debate between optimists and pessimists that has become embodied by political scientists Kenneth Waltz and Scott Sagan. Finally, it considers the concept of nuclear latency whereby some states can achieve a position where they could—if they chose to do so—transform a civilian nuclear capability into a weaponised one in a fairly short space of time.