This chapter describes the evolution of a disaster ecological framework for portraying the impact of disasters on human populations. It begins with a detailed look at exposure to hazards-categorized by type, intensity, time, and place factors. Globally, the cumulative impact of disasters can be estimated using multiple measures. Directly relevant to the field of disaster psychiatry, the degree of psychosocial impact varies by disaster type and generally increases with increasing magnitude and frequency of disaster occurrence. The public health consequences of disasters can be assessed in terms of mortality, morbidity and disruption of health care infrastructure. Disaster impact on citizens and whole populations varies by individual and family characteristics such as age, gender, race/ethnicity, education, occupation, employment status and income. The scope and magnitude of disasters are associated with the extent of disruption of health and social services.