Primary care plays a central role in the provision of health care, and is an organizing feature for health care delivery systems in most Western industrialized democracies. For a variety of reasons, however, the practice of primary care has been in decline in the U.S. This paper reviews key primary care concepts and their definitions, notes the increasingly complex interplay between primary care and the broader health care system, and offers research priorities to support future measurement, delivery and understanding of the role of primary care features on health care costs and quality.