This dissertation uncovers the importance of religion in shaping the United States' international development programs, with a particular focus on development in Latin America. It locates the origins of U.S. development efforts in the work of early twentieth-century Christian missionaries who launched agricultural improvement projects in rural regions of the world. In their endeavor to raise rural standards of living, these agricultural missionaries transformed local agricultural economies and natural environments through the introduction of new crops and breeds of livestock. They also built close relationships with governments in both their host countries and the United States through the sharing of scientific, political, and cultural expertise. When the U.S. government began to embrace development as a foreign policy objective during the Cold War, state actors turned to agricultural missionaries for advice and used their projects as models for their own development programs. Missionaries consistently worked to advocate on behalf of a small-scale, agriculture-focused, community development approach. When government-run programs increasingly took an approach that favored large-scale modernization and top-down economic aid, agricultural missionaries distanced themselves from the state and its model of development, which they criticized for exacerbating political instability, socioeconomic inequality, and environmental problems. In short, agricultural missionaries played central roles in creating, carrying out, and contesting the U.S. international development project.