This symposium addresses the complications encountered by medical planners when confronted by the use or threat of the use of weapons of mass destruction. The types of chemical warfare agents (CWA), their principal target organs, and physiological effects are discussed. Reviewed here are the uses of CWA in 20th century warfare and terrorism, with emphasis on 5 cases: (1) use of sulfur mustard during World War I; (2) use by Italy against Ethiopia; (3) use in the Sino-Japanese War; (4) relatively well-studied use in the Iran-Iraq conflict; and (5) the use of sarin in the Tokyo subway terrorist incident. Symposium contributors reviewed the additional physiological and psychological consequences of their use and threat of use. Results from training and simulation are discussed. Conclusions are presented derived from the analysis of these historical situations