Weapons of Mass Destruction received attention before the events of September 11th 2001, but much more concerted interest dates to March 1995 when Aum Shinrikyo, a Japanese religious cult carried out an attack on the Tokyo subway, killing twelve and injuring five thousand. But for an element of technical incapacity, the casualty figures would have been much greater. WMD terrorism is a low-probability, but it is also a high-consequence threat, for the harm caused by even one successful act would be profound, not only in terms of lives lost. The conventional, low-technology terrorism of the past has exercised a social and political impact far out of proportion with the casualties it has caused. The massive, indiscriminate destruction caused by an act of WMD terrorism similarly would have disproportionate social, political, economic and strategic effects. Twenty-nine articles are republished here exploring the issue of WMDs and key questions raised about terrorism, its definition, objects, motivation, whether there is a different species of it emerging in the aftermath of the close of the Cold War, preparedness and the means of prevention.