Variation in use of health care is ubiquitous in the United States. It is attributable to exogenous differences in supply of medical resources; to identified and unidentified economic, social, and cultural factors; and to the idiosyncratic beliefs of physicians. It is perpetuated by the parochial character of much clinical practice. Patients in high-intensity areas do not appear to have better health outcomes: Much care is "flat of the curve." A more robust scientific foundation for clinical decisions could help to reduce variations, but major reform of health care financing is probably necessary to achieve substantial improvement in the organization and delivery of care.