This article reviews the existing literature on the efficacy of primary care with an emphasis on the evaluation of primary care for vulnerable populations: groups whose demographic, geographic, or economic characteristics impede or prevent their access to health care services. A significant portion of the literature derives from studies of poor and underserved populations. However, to construct a more complete evaluation of primary care services, the authors cite literature that has examined both advantaged and disadvantaged populations. Even then the literature is incomplete, at best. The article describes a definition of primary care suitable for policy analysis and formulation, reviews evidence on the efficacy of care that meets that definition, and concludes that widespread use of primary care services is likely to result in improved patient satisfaction and health status.