Rapid urbanization is a hallmark of our age. In 1950 fewer than one out of three of the world's people lived in cities. By 2050, according to United Nations projections, the figure will be almost two out of three. Meanwhile the world's population will have more than tripled. In 1950, 750 million people lived in urban areas; by 2050, demographers project, 6.3 billion will--more than eight times as many. For the most part, farmers have kept up with the increase in urban numbers, growing more food and distributing it in the newly expanding cities. Water has a poorer record. Cairo, Buenos Aires, and San Antonio; Dhaka, Istanbul, and Port-au-Prince; Miami, Manila, Monrovia, Mumbai, and Mexico City--all have greatly expanded, and all have failed to keep up with the demand for clean, plentiful water.