Despite significant reductions in nuclear arsenals after the Cold War, the world’s nuclear nations still are pointing thousands of warheads at each other, using systems that are vulnerable to human error, miscalculation, or cyberwarfare. Over the past decades, humankind has come hair-raisingly close in several well-documented instances to unleashing full-scale nuclear war. The logic of arms races makes it very hard for nations to resist the temptation of modernizing and building up their arsenals. On the other hand, new designs for nuclear reactors – whether for fission or fusion processes – hold out the tantalizing possibility of safe, cheap, and abundant electricity. Nuclear-generated electricity does not release greenhouse gases, and therefore could play a key role in creating a sustainable energy system over the coming century.