The urbanization of nineteenth century America led to enormous changes in American criminal justice, as the rise of this dramatically more complex kind of human settlement posed new problems for legal regulation. Some of those problems are familiar. Many reformers emphasized the way cities eroded traditional controls on predatory crime, and they viewed modern police forces, public prosecution, and the modern penitentiary as a means of substituting formal social control for the informal controls of the past. But cities posed a different problem as well. In the city people made their homes in dense mixed-use environments that had not yet been sorted out and segregated along the lines of the modern metropolis, and when they ventured out of them they came together in the crowded streets, squares, and parks that proliferated in the nineteenth century. This complex environment made new demands on their behavior, as conduct that would have bothered no one in sparsely occupied rural spaces became problematic in the densely shared environments of the city. This change did not involve the collapse of old strategies for controlling familiar forms of bad behavior; it involved a shift in what sort of behavior counted as “bad” in the first place. The continued evolution of the urban environment, in turn, depended upon the ability of criminal justice institutions to grapple with these challenges. Shared environments require those who use them to develop and enforce rules to regulate the sharing.