Health care depends on people. It is the health workforce — doctors, nurses, pharmacists, lab technicians, and nursing assistants, to mention a few — that, in large measure, determine the quality and effectiveness of any health enterprise. The nature of the health workforce was integral to the health care reform debates of the early 1990s and will surely be central in proposals to improve the quality, accessibility, and cost of U.S. health care in the future. Therefore, as we enter a new period of health reform deliberations and as we face the inevitability of an aging population, it is important to consider the problems and potential remediation necessary for the health workforce of the future. This article focuses particularly on physicians and nurses while recognizing the importance and frequently parallel concerns that arise across all health care disciplines. Understanding how the composition, capacity, and activities of a health care workforce must change as part of health reform requires an understanding of the context in which health professionals practice. What are the factors that govern health care today, and what are their impact on the health workforce?