Law plays a critical role in all stages of a public health emergency, including planning, response, and recovery. Public health emergencies introduce health concerns at the population level through, for example, the emergence of a novel infectious disease. In the United States, at the federal, state, and local levels, laws provide an infrastructure for public health emergency preparedness and response efforts: they grant the government the ability to officially declare an emergency, authorize responders to act, and facilitate interjurisdictional coordination. Law is perhaps most visible during an emergency when the president or a state's governor issues a disaster declaration establishing the temporal and geographic parameters for the response and making financial and other resources available. This legal authority has increasingly been used during the last decade.