Disasters of industrial origin are usually overt, but sometimes their source is silent and their occurrence diluted in time. Thus, the ascertainment of the health impact is not the sole epidemiologic task; epidemiology is often needed to identify the disaster source; in addition, a postdisaster scenario may represent an unfortunate, unplanned experimental setting from which scientific knowledge and public health achievements can be drawn. In the aftermath of a disaster, three sets of illness determinants ought to be considered: the relevant exposure; the stressful experience of the population; the response measures. Ecological, geographic, and personal indicators might be used to ascertain individuals' exposure. Relevant health outcomes (early and long-term) are either exposure-related or stress-related (mental and physical). A cohort approach should be sought to avoid major selection/information biases and facilitate later studies. Early planning is of paramount importance to identify study goals, problems, and resource requirements.