This chapter discusses changes in criminal law in the twentieth century, covering the decriminalization of sex and vice, defendants’ rights, the death penalty, and crime waves and the national response. As the federal government grew in size in the twentieth century, and as the federal statute book grew along with it, a whole new array of federal crimes came into existence. Income tax evasion or fraud was one of these—it was obviously no crime at all before the federal income tax law. Every regulatory law created a new federal crime: violating the food and drug law, or stock fraud under the SEC law, or killing a black-footed ferret, under the Endangered Species Act. Under J. Edgar Hoover, the Federal Bureau of Investigation became a major crime-fighter, at least with regard to crime that crossed state lines.