The nuclear programs of the Democratic People's Republic of Korea (DPRK), Iran, and Pakistan provide the most visible manifestations of three broad and interrelated challenges to the nuclear nonproliferation regime. The first is so-called latent proliferation, in which a country adheres to, or at least for some time maintains a façade of adhering to, its formal obligations under the Nuclear Nonproliferation Treaty (NPT) while nevertheless developing the capabilities needed for a nuclear weapons program. That country can then either withdraw from the NPT and build actual weapons on short notice, or simply stay within the NPT while maintaining the latent capability for the rapid realization of nuclear weapons as a hedge against future threats. This was the path followed by the DPRK with its plutonium program and one that is likely being followed by Iran and more subtly by others. The second broad challenge is first-tier nuclear proliferation, in which technology or material sold or stolen from private companies or state nuclear programs assists nonnuclear weapons states in developing illegal nuclear weapons programs and delivery systems. The third challenge--the focus of this article--is second-tier nuclear proliferation, in which states in the developing world with varying technical capabilities trade among themselves to bolster one another's nuclear and strategic weapons efforts.