This article reviews the current status of theory and research concerning the social determinants of health. It provides an overview of current conceptualizations and evidence on the impact of various social determinants of health. The contributions of different disciplines--epidemiology, sociology, political economy, and the human rights perspective--to the field are acknowledged, but profound gaps persist in our understanding of the forces that drive the quality of various social determinants of health and why research is too infrequently translated into action. Many of these gaps in knowledge concern the political, economic, and social forces that make implementation of public policy agendas focused on strengthening the social determinants of health problematic. The author identifies the areas of inquiry needed to help translate knowledge into action.