The twentieth century saw the creation and development of weapons of mass destruction (WMD): nuclear, chemical, and biological. In the following article, Ryukichi Imai, distinguished research fellow at the Institute for International Policy Studies and visiting professor at Kyorin University, Japan, describes the history of WMD and the international treaties that have been negotiated on their non-proliferation. He focuses on the history of the nuclear bomb from its first tests, to the nuclear arms race and the subsequent deterrence. He goes on to examine the issues surrounding the nuclear hot-spots of South Asia and North Korea, as well as the feasibility and effects of chemical and biological weapons. He argues that the suicide plane attacks on 11 September changed the very concept of WMD and that any future threat of a mass destruction is likely to come from terrorists beyond the reach of governments.