The dangers posed by weapons of mass destruction have come to occupy center stage in international politics. The term “weapon of mass destruction” (WMD) is used to characterize a variety of weapons that share two key features: their potential for large-scale destruction and the indiscriminate nature of their effects, notably against civilians. There are three major types of WMD: nuclear weapons, chemical warfare agents, and biological warfare agents. In addition, some analysts include radiological materials, as well as missile technology and delivery systems such as aircraft and ballistic missiles. The dangers posed by nongovernmental actors and non-state terrorist networks, through clandestine programs and black-market sales of weapons and related technologies, are also discussed. Fears of the terrorist use of WMD have greatly increased in the United States and around the world.