One of the major concerns of the ``greenhouse effect'' is the impact of global warming on the supply of and demand for water. Beginning as a project of the Committee on Climate of the American Association for the Advancement of Science, it focuses on the United States because of the wealth of climactic and hydrologic data and models available, and also because of the great diversity of the nation's water problems. It is equally concerned with the effects of climactic variability in the face of changing water uses and demands, as with possible long-term climactic change. Recommendations are made for scientists, public bodies, private persons and enterprises.