People use health care services for many reasons: to cure illnesses and health conditions, to mend breaks and tears, to prevent or delay future health care problems, to reduce pain and increase quality of life, and sometimes merely to obtain information about their health status and prognosis. Health care utilization can be appropriate or inappropriate, of high or low quality, expensive or inexpensive. The study of trends in health care utilization provides important information on these phenomena and may spotlight areas that may warrant future indepth studies because of potential disparities in access to, or quality of, care.