For 2 decades, health planners have forecasted impending physician surpluses, and policy decisions related to medical schools and residency programs have been based on such expectations. However, these much-heralded surpluses never materialized, and a growing body of data and opinion now point in the other direction. The question at the forefront is whether the United States is instead headed for a physician shortage. What is the evidence? This paper reviews the trends that link economic growth to health care spending and to the demand for physicians. It assesses the current environment by examining trends in the characteristics of clinical practice, signals from the medical market, and recent experiences of physician shortages in other English-speaking countries; it also discusses why past forecasting approaches may have failed. Taken together, this body of information indicates that physician shortages are emerging and that they will probably worsen over the next 2 decades. By 2020 or 2025, the deficit could be as great as 200,000 physicians--20% of the needed workforce. If remedies are to be found, the nature of the problem must be appreciated, and a consensus to solve it must be reached.