Constructing Economic Science demonstrates how an existing public discourse, political economy, was transformed in the early twentieth century into a new university discipline: economics. This change in location brought about a restructuring of economic knowledge. Finance, student numbers, curricula, teaching, new media, and the demands of employment all played their part in shaping economics as it is known today. It was broadly accepted in the later nineteenth century that industrialising economies required the skilled and specialised workforce that universities could provide. Advocacy for the teaching of commercial subjects was widespread and international. In Cambridge, Alfred Marshall was alone in arguing that economics, not commerce, provided the most suitable training for the administration and business of the future; and in 1903 he founded the first three-year undergraduate economics programme. This was by no means the end of the story, however. What economics was, how Marshall thought it should be taught, had by the 1920s become contested, and in Britain the London School of Economics gained dominance in defining the new science. By the 1930s, American universities had already moved on from undergraduate to graduate teaching, whereas in Britain university education remained focussed upon undergraduate education. At the same time, public policy was reformulated in terms of economic means and ends—relating to postwar reconstruction, employment, and social welfare—and international economics became American economics. This study charts the conditions that initially shaped the “science” of economics, providing in turn a foundation for an understanding of the way in which this new language itself subsequently transformed public policy.