Agents of chemical warfare continue to pose a threat to human life. Organophosphorus compounds are possibly the best known and most used agents in recent times. These are known to produce acute ill health and death and, probably equally important, many diverse delayed effects, many of which are not clinically nor pathologically well defined. The immediate and delayed effects of organophosphorus compounds, in particular, and those of other known agents of chemical warfare, such as mustard gas, Lewisite, phosgene, cyanides and the newer crowd control agents, are reviewed. Environmental sequelae of these agents are gaining importance as probable causes of chronic ill health amongst those living in regions where these agents have been used. The need to study the pattern of disease in exposed populations is emphasised.