Population Health Implications of the Affordable Care Act is the summary of a workshop convened in June 2013 by the Institute of Medicine Roundtable on Population Health Improvement to explore the likely impact on population health improvement of various provisions within the Affordable Care Act (ACA). This public workshop featured presentations and discussion of the impact of various provisions in the ACA on population health improvement. Several provisions of the ACA offer an unprecedented opportunity to shift the focus of health experts, policy makers, and the public beyond health care delivery to the broader array of factors that play a role in shaping health outcomes. The shift includes a growing recognition that the health care delivery system is responsible for only a modest proportion of what makes and keeps Americans healthy and that health care providers and organizations could accept and embrace a richer role in communities, working in partnership with public health agencies, community-based organizations, schools, businesses, and many others to identify and solve the thorny problems that contribute to poor health. Population Health Implications of the Affordable Care Act looks beyond narrow interpretations of population as the group of patients covered by a health plan to consider a more expansive understanding of population, one focused on the distribution of health outcomes across all individuals living within a certain set of geopolitical boundaries. In establishing the National Prevention, Health Promotion, and Public Health Council, creating a fund for prevention and public health, and requiring nonprofit hospitals to transform their concept of community benefit, the ACA has expanded the arena for interventions to improve health beyond the "doctor's" office. Improving the health of the population - whether in a community or in the nation as a whole - requires acting to transform the places where people live, work, study, and play. This report examines the population health-oriented efforts of and interactions among public health agencies (state and local), communities, and health care delivery organizations that are beginning to facilitate such action.