"Passage to Modernity" is a history of the American experts who sought to "modernize" the post-1945 world. It traces the origins of postwar development to India and examines how one group of experts, social scientists, sought a passage to modernity and a proving ground for their expertise in the practice of social science abroad. The postwar project to "modernize" the developing world has been chronicled as the rise of an intellectual paradigm, "modernization theory," made in U.S. universities to fit U.S. geopolitical needs. Postwar American intellectuals, however, also operated on a world stage of overseas fieldwork and transnational collaboration with foreign academics and governments. This thesis explores how India came to be seen by anthropologists, economists, and planners as a model terrain on which to fashion plans for developing emerging nations during the Cold War. Before a canon of "modernization theory" could take form, specific locales had to be imagined as being made modern. Of all the places in the world selected as sites for applied knowledge, none was more important in the 1950s than India. A wave of "India projects"--from the Chicago civilization project to the Cornell village studies to the MIT development program--produced new transnational social networks and new ideas about the place of tradition, the nation state, and academic expertise in the making of modernity. They also provoked conflicts over political control of development and the appropriate uses of social science. By 1960, Indian experience had transformed U.S. intellectuals into modernization experts but left them chastened about the possibilities of applied social science. This dissertation argues that the next decade's modernization theory was in part a retreat from the contentious ground of practice. Historians have characterized Cold War U.S. intellectuals as fixated on the world yet have rarely depicted them as actors in the world. The history of the India projects underscores the crucial role that international relationships and places beyond America's borders played in the intellectual life and professionalization of American social scientists. By uncovering the transnational history of development, historians may begin to abandon the false dichotomy of American hegemony and foreign resistance.