Abstract : With the threat of terrorism, concerns about the use of radiological and nuclear weapons are high. Billions of dollars have been appropriated in the United States for the development and deployment of new technologies for monitoring nuclear weapons and materials within the United States and overseas (Medalia, 2009). The U.S. Congress has enacted laws requiring that containers be scanned by imaging and radiation detection equipment before loading onto vessels in foreign ports (9/11 Commission Act, 2007), with the intention of preventing terrorists from smuggling nuclear weapons or fissile materials [special nuclear material (SNM)] into the country. Nuclear detection can impede a terrorist nuclear attack in two ways: deterrence and defense. A successful detection capability, in combination with the inherent difficulties of constructing an effective nuclear weapon, could convince adversaries that any attempt to launch a nuclear attack will fail or that such an attack would be too complex to execute. These consequences could dissuade them from action. If, however, deterrence fails, detection systems would be needed, allowing the discovery of materials and interdiction to prevent their use. The detection technologies could be used to search vehicles, structures, containers, and possibly individuals for SNM or weapons at border crossings, transit routes, and other locations of concern.