The underlying premise of epidemiology is that disease does not occur at random, but rather in patterns that reflect the operation of underlying factors. Epidemiologic methods determine the extent of disease in the community, help to examine the natural history and prognosis of disease, assist in identifying associations and potential etiology, that is, causes, of a disease and risk factors for disease, aid in the evaluation of new preventive and therapeutic measures and new modes of health care delivery, and help provide a foundation for developing public health policy and regulatory decisions relating to environmental problems. This video reviews terms commonly used in epidemiology and describes types of epidemiologic studies,