This article defines population health as the health outcomes of a group of individuals, including the distribution of such outcomes within the group. Population health includes health outcomes, patterns of health determinants, and policies and interventions that link these two. Attention to social and environmental, as well as medical, determinants of health is essential. The population health lens can be used at the individual, practice, institutional, and community levels. The need for primary care to engage in population health stems from the importance of social and environmental factors, the nature of primary care, and contextual changes.