Equity in health and health care have become important priorities for the world. If efforts at achieving equity are to have any basis in evidence concerning which strategies are likely to work, a research agenda is necessary. An adequate research agenda requires a knowledge of what the problem is, an understanding of the genesis and correlates of the problem, methods to measure these correlates, and rigorous testing of alternative explanations and interventions. This article presents a working definition of equity in health and health services, a conceptual framework in which to view the various types of influence on health and distribution of health in populations, a summary of evidence on the effects of some of these categories, and a research agenda for guiding efforts to improve knowledge on which to base interventions that enhance the attainment of equity. Because of their relative neglect in the existing literature on equity in health, the special roles of political forces and of primary care as a particularly key element of health services are stressed.