The authority to act to protect and promote the public's health has emerged over the course of our Nation’s history, and has enabled many of modern public health’s greatest achievements. Today, public health agencies use their authority to promote health, prevent the spread of infectious disease, protect against environmental hazards, advance equity and assure local public health infrastructure and health services. During declared emergencies, state and federal authorities have broader powers to act to assist governments, suspend or modify legal requirements, pass and enforce law and expend funds for the health, safety and welfare of society. Recently, some states have moved to limit public health powers. Such efforts pose a mounting challenge for public health’s response to COVID-19 and future threats.