Health care reform proposals that seek to increase access for almost 40 million uninsured Americans will require an expansion of primary care. Simply expanding available primary care services is likely to be prohibitively expensive. Developing a rational and efficient strategy for the provision of primary care to all Americans will require understanding of the relationships among primary care and access, costs, and quality--relationships that are often obscured by the complexity of the US health care system. Those relationships are examined to identify some key areas of research needed to inform policy development and improve primary care services. Despite significant gaps in our knowledge, the evidence strongly suggests that restructuring and strengthening the role of primary care practitioners in the health care system will facilitate access to affordable, high-quality health care for all Americans.