Posttraumatic stress disorder (PTSD) was officially introduced into psychiatric nomenclature in 1980, when it was incorporated into DSM-III. There is as yet little empirical research on the validity of the diagnosis. Literature on disasters, civilian and wartime, and on more ordinary stressful life events does not support the view that extreme stressors form a discrete class of stressors in terms of the probability of psychiatric sequelae or the distinctive nature of subsequent psychopathology. Extraordinary stressors are like more ordinary stressful events with respect to their complex differential effects upon individuals. Personal characteristics and the nature of the social environment modify the likelihood and form of the response of individuals to all types of stressors.