Public health practice involves all that is done to improve health, including activities, programs, infrastructure, interventions, services, and processes that prevent disease and promote health for all people, everywhere. Public health practice involves the delivery of public health services at the global, national, state, and local levels—everything done to prevent disease and promote health. This chapter first outlines the structure of the public health systems in the United States and abroad. It discusses the organization and scope of public health practice locally and globally. The chapter describes the three core functions of public health practice: assessment, policy development, and assurance. It discusses assessment as an essential tool to systematically track health in populations. The chapter describes the surveillance techniques for ongoing monitoring of health. It discusses policy development as a means to address causes of health, and the techniques for ongoing assurance of the impact of policies, programs, and services.