This article models how population growth and climate change will affect water availability for all cities in developing countries with >100,000 people. These cities had 1.2 billion residents in 2000, 60% of the urban population of developing countries and, according to our demographic projections, will account for 74% of all urban growth globally from 2005 to 2050 (1). We describe the magnitude and general patterns of the global challenge of water availability for urban residents, recognizing that such a global approach cannot account for each city's particular circumstances.