Terrorist use of radioactive nuclear materials is a serious threat for mass destruction or disruption of civil and military activities. Most worrisome is the use of nuclear devices to cause massive casualties to people and damage to structures. Fortunately, the procurement of adequate material and the engineering design, construction, and transportation and triggering of a nuclear weapon are all difficult problems for terrorist organizations. More likely is a device that combines radioactive materials with conventional explosives to make a radiological dispersion device, commonly called a "dirty bomb." The procurement of nuclear materials for this purpose, the construction of the bomb and its use are all easier than for a nuclear weapon. Fortunately, the effects from the use of a radiological weapon would be much smaller than from a nuclear device, although they could still be very disruptive. Thus, it is important to detect the transport of nuclear weapons and radiological dispersion devices and the materials for their construction. These materials emit gamma rays or neutrons, which can be detected to show the presence and amounts of such materials.