An estimated 99 million Americans live with a chronic illness. Meeting the needs of this population is one of the major challenges facing the U.S. health care system today and in the future. Dozens of studies, surveys, and audits have revealed that sizable proportions of chronically ill patients have not received effective therapy and do not have optimal disease control. The consistent findings of generally substandard care for many chronic conditions have spurred proposals that care be shifted to specialists or disease management programs. Published evidence to date does not indicate any clear superiority of these alternatives to primary care. The defining features of primary care (that is, continuity, coordination, and comprehensiveness) are well suited to care of chronic illness. A rapidly growing body of health services research points to the design of the care system, not the specialty of the physician, as the primary determinant of chronic care quality. The future of primary care in the United States may depend on its ability to successfully redesign care systems that can meet the needs of a growing population of chronically ill patients.