Preventing nuclear war and avoiding catastrophic climate change are two of the most basic challenges facing human civilization in the twenty-first century. While these are separate issues, these challenges are linked in several ways, and both may be affected by the future of nuclear energy. For nuclear energy to provide any substantial part of the low-carbon energy needed in the second half of the twenty-first century would require dramatic growth. This chapter provides an overview of the constraints and risks of nuclear energy growth on that scale, and the necessary steps to address them. In particular, use of nuclear energy at that scale would place unprecedented demands on global systems for verification, control, and security for weapons-usable nuclear materials. Deep reductions in nuclear arms and their eventual prohibition will also require new approaches to managing the vast global stocks of weapons-usable nuclear materials. Politically, nuclear energy may not be able to grow on the scale required unless governments and publics are confident that it will not contribute to the spread of nuclear weapons, creating another link between climate mitigation and nuclear nonproliferation and disarmament.