The construction of tens of thousands of large dams across the planet’s surface brought about one of the largest biophysical transformations of the twentieth century and has irrevocably altered human-environment relations. The geopolitical dimensions of this “concrete revolution” have remained largely hidden. The history of large dams and more generally river basin development is simultaneously environmental, social, technical and geopolitical. This book focuses on the activities of the United States government, in particular the Bureau of Reclamation, America’s premier water development agency, to exercise and disseminate technical expertise regarding large hydroelectric dams and river basin planning and development to the world’s “underdeveloped regions” from the 1930s to the 1970s. The Bureau’s water resource development activities, which ranged from short-term consultations to intensive multi-year programs, were deeply influenced by the imperatives of US foreign policy during the Cold War era. Detailed cases presented in the book—including Bureau interventions in China, Lebanon, Ethiopia and the Mekong Basin—underscore how the geopolitical dynamics of the Cold War facilitated an alignment of economic and technical networks of development that were highly favorable to the dissemination of large dams. Large dams and other technology-centered development projects are never purely technical undertakings whose successes or failures hinge on the ingenuity of the engineers who design and build them or the motivations of state officials who fund and promote them. The lessons of the history presented here are that large dams and river basin planning are complex hybrids of nature, technology and society.