Physician shortages and their implications for required increases in the physician population are matters of considerable interest in many health care systems, especially in the light of the widespread phenomenon of population aging. To determine the extent to which shortages exist, one needs to study the population of users of physician services as well as that of the physicians themselves. The authors study both, using the province of Ontario, Canada, as an example. The user population is projected and the implications for requirements calculated, conditional on given utilization rates. On the supplier side, the age and other characteristics of the (active) physician population are examined and patterns of withdrawal investigated. The necessary future growth of supply is calculated, assuming alternative levels of present shortages. The effects of population change on requirements are found to be smaller in the future than in the decade 1981–1991, in the aggregate, not far from the effects in 1991–2001, but highly variable among different categories of physicians.