Nuclear weapons pose a particularly destructive threat. Prevention of the proliferation and use of nuclear weapons is urgently important to public health. “Horizontal” proliferation refers to nation-states or nonstate entities that do not have, but are acquiring, nuclear weapons or developing the capability and materials for producing them. “Vertical” proliferation refers to nation-states that do possess nuclear weapons and are increasing their stockpiles of these weapons, improving the technical sophistication or reliability of their weapons, or developing new weapons. Because nation-states or other entities that wish to use or threaten to use nuclear weapons need methods for delivering those weapons, proliferation of delivery mechanisms must also be prevented. Controlling proliferation—and ultimately abolishing nuclear weapons—involves national governments, intergovernmental organizations, nongovernmental and professional organizations, and society at large.