A model is presented to illustrate some areas in which epidemiological thinking may be applied to health services research. The model is neither exhaustive nor exclusive, for other areas not specifically described as within the model are also a part of the domain of epidemiology, and several aspects of the model are shared with researchers in other disciplines. Selected examples of epidemiological applications are reviewed; these include: (1) the need for health care services, (2) the use of health services, (3) the impact of health programs on utilization patterns and on health status, and (4) the impact of therapeutic services on outcomes for specific diseases. Additional health services issues that are relevant to epidemiology are suggested.