Half the world’s population now lives in urban areas, and the trend toward greater urbanization shows no signs of abating. Among the many diffi- culties facing planners seeking to accommodate continued development, finding adequate water supplies will be among the most critical. As Jerry Anthony points out in his paper in this issue, one in five people in the developing world does not have access to safe drinking water, and efforts at planning for water over the last three decades have not improved that desperate condition. There are reliable estimates that more than a billion people get water for drinking, washing, and cooking from sources polluted by human and animal feces. We are unable to provide adequate water for drinking, sanitation, or agriculture for four to five billion people, yet within a few decades the world’s population will double or triple. Global climate change will likely have its most serious impacts on precipitation patterns, reducing water in many parts of the world.