This chapter provides an overview of epidemiologic research on the mental health effects of major disasters. It begins by examining disaster typology and then proceeds to examine various outcomes of disasters, and predictors such as preexisting characteristics, exposure status, and time frame. The chapter also critically reviews other predictors of potential relevance for post disaster settings. Predicting mental health outcomes of disasters is vital to directing mental health resources that may be scarce in postdisaster settings. Community response to disaster may affect mental health problems that may be reduced by an outpouring of community support. The first task in responding to mental health effects following disasters is to differentiate psychiatric illness from distress, because these two entities generally require different approaches and interventions tailored to their needs. Post-traumatic stress disorder (PTSD) and other disorders need psychiatric evaluation and treatment, because effective treatments are available.