Detection of nuclear weapons and special nuclear material (SNM) is crucial to thwarting nuclear proliferation and terrorism and to securing weapons and materials worldwide. Congress has funded a portfolio of detection R&D and acquisition programs, and has mandated inspection at foreign ports of all U.S.-bound cargo containers using two types of detection equipment. Nuclear weapons contain SNM, which produces unique or suspect signatures that can be detected. It emits radiation, notably gamma rays (high-energy photons) and neutrons. SNM is very dense, so it produces a bright image on a radiograph when X-rays or gamma rays are beamed through a container in which it is hidden. Some detection technology is advancing faster than many have expected. It is easier and less costly to accelerate a program in R&D than in production. This book emphasizes the ongoing improvement in detection capabilities which produce uncertainties for terrorists that will increase over time, adding deterrence beyond that of the capabilities themselves.This book consists of public documents which have been located, gathered, combined, reformatted, and enhanced with a subject index, selectively edited and bound to provide easy access.