Statutes, regulations, and litigation are pivotal tools for creating conditions for people to lead healthier and safer lives. Law can educate, create incentives, and deter; mandate safer product design and use of property; and alter the informational, physical, or economic environment. This article defines public health law as the power and duty of the state to ensure conditions for people to be healthy and limitations on the state's power to constrain autonomy, privacy, liberty, and proprietary interests of individuals and businesses. The 5 essential characteristics of public health law discussed are (1) the government's responsibility to defend against health risks and promote the public's health; (2) the population-based perspective of public health, emphasizing prevention; (3) the relationship between government and the populace; (4) the mission, core functions, and services of the public health system; and (5) the power to coerce individuals, professionals, and businesses for the community's protection. JAMA. 2000;283:2837-2841