The international boundary between the United States and Mexico spans more than 1,900 miles. Along much of this international border, water is what separates one country from the other. Border Water provides a historical account of the development of governance related to transboundary and border water resources between the United States and Mexico in the last seventy years. This work examines the phases and pivot points in the development of U.S.-Mexico border water resources and reviews the theoretical approaches and explanation that impart a better understanding of these events. Author Stephen Paul Mumme, a leading expert in water policy and border studies, describes three important periods in the chronology of transboundary water management. First, Mumme examines the 1944 Water Treaty, the establishment of the International Boundary and Water Commission (IBWC) in 1945, and early transborder politics between the two governments. Next, he describes the early 1970s and the rise of environmentalism. In this period, pollution and salinization of the Colorado River Delta come into focus. Mumme shows how new actors, now including environmentalists and municipalities, broadened and strengthened the treaty’s applications in transboundary water management. The third period of transborder interaction described covers the opening and restricting of borders due to NAFTA and then 9/11. Border Water places transboundary water management in the frame of the larger binational relationship, offering a comprehensive history of transnational water management between the United States and Mexico. As we move into the next century of transnational water management, this important work offers critical insights into lessons learned and charts a path for the future.