The diffusion and U.S. geographic distribution of primary care physicians are discussed in three papers. The literature on the diffusion issue is reviewed in the first paper. After introducing diffusion concepts, measures, and problems, current evidence for the diffusion of physicians are assessed, and efforts to forecast future needs for physicians in health manpower shortage areas are considered. In the second paper, forecasts are presented of population-to-primary care physicians ratios in U.S. counties over the 1982-1994 period. The forecasts are based on an econometric model developed by the Modeling and Research Branch of the Office of Data Analysis and Management, a federal agency. These forecasts do not provide estimates of physician needs in shortage areas because the majority of shortage areas are not whole counties. Therefore, the third paper went beyond the county-level forecasts to project rates of change in population-to-physician ratios for subcounty shortage areas and to calculate future needs based on the current shortage area designation criteria. Assumptions underlying the forecasts of the econometric model are also identified, with attention to projections of the supply of physicians at the national level and the county level projections of population and income. (SW)