The public health system serves the entire American population by preventing disease, prolonging life, and promoting health by providing the safety net of the 10 essential public health services. These systems use the basic science of public health known as epidemiology to help assess the health needs of a community, and this information can then be used to develop interventions that promote health and prevent disease. Primary public health activities include assessment, policy development, and assurance. Public health regularly focuses on primary, secondary, and tertiary prevention throughout every community. The opportunities for increased collaboration between public health and population health are many. This chapter examines the fields of public health and population health and how they utilize epidemiology to manage the health of populations.