The proliferation of nuclear, chemical, and biological weapons (collectively known as weapons of mass destruction, or WMD) and the potential acquisition and use of WMD against the world by terrorists are extremely serious threats to international security. These threats are complex and interrelated. There are myriad routes to weapons of mass destruction--many different starting materials, material sources, and production processes. There are many possible proliferators--threshold countries, rogue states, state-sponsored or transnational terrorists groups, domestic terrorists, and even international crime organizations. Motives for acquiring and using WMD are similarly wide ranging--from a desire to change the regional power balance, deny access to a strategic area, or alter international policy to extortion, revenge, or hate. Because of the complexity of this threat landscape, no single program, technology, or capability--no silver bullet--can solve the WMD proliferation and terrorism problem. An integrated program is needed that addresses the WMD proliferation and terrorism problem from end to end, from prevention to detection, reversal, and response, while avoiding surprise at all stages, with different activities directed specifically at different types of WMD and proliferators. Radiation detection technologies are an important tool in the prevention of proliferation. A variety of new developments have enabled enhanced performance in terms of energy resolution, spatial resolution, predictive modeling and simulation, active interrogation, and ease of operation and deployment in the field. The radiation properties of nuclear materials, particularly highly enriched uranium (HEU), make the detection of smuggled nuclear materials technically difficult. A number of efforts are under way to devise improved detector materials and instruments and to identify novel signatures that could be detected. Key applications of this work include monitoring for radioactive materials at choke points, searching for nuclear materials, and developing instruments for response personnel.