The health of Arizona’s citizens depends on an adequate supply of physician services to meet their health care needs. The supply of physician services is determined by the number of practicing physicians, the mix of physician specialties, and by the productivity of physicians. This is the first of two reports on the supply of physician services in Arizona. This report describes the results of a comprehensive review of the current physician workforce in Arizona and of the trends in the number, specialty composition, and geographic distribution of the physicians. A subsequent report (Part II) combines the numbers of physicians with measures of productivity to estimate the supply of physician services.