Beginning after the end of Reconstruction, this chapter looks at the ways in which the police power emerged to facilitate an increasingly bold project of regulation. Key Supreme Court decisions supported the use of the police power to undertake and implement the objectives of a growing economy and a widening sphere of government. State power accompanied expanding national power and all levels of government tackled myriad persistent and new problems. In a case from the early twentieth century, for example, the Court upheld a vaccine requirement as a reasonable exercise of the public health authority of the state. Regulatory power was called into question by the Supreme Court’s Lochner-era decisions, but even this two-decades-long movement did not seriously threaten the ability of state governments to carry out ambitious regulatory agendas. Significantly, the Court put its imprimatur on the government’s zoning power in key cases from the late 1920s. And though the Court would message to the states that there were limits on how far they could go in restricting property rights, through doctrines such as “regulatory takings,” what emerged by the end of World War II was a robust conception of the state police power, one that gave government a wide sphere of action and authority to protect the general welfare.