Nineteenth-century America witnessed a period of tremendous growth and change as cities flourished, immigration swelled, and industrialization spread. This setting allowed prostitution to thrive and professionalize, and the visibility of such “immoral” activity required Americans to seek a new understanding of morality. Current literature commonly considers prostitution as immediately declared a “social evil” or briefly mentions why Americans assigned it such a role. While correct that it eventually did become a “social evil,” the evolution of discourse relating to prostitution is a bit more complex. This thesis provides a survey of this evolution set against the changing American understanding of science and morality in the nineteenth century. By tracing the course of American thought on prostitution from necessary to social evil, this thesis contributes to a growing understanding of a marginalized group of people and America’s view of national morality.