Half of the world's inhabitants now live in cities. In the next twenty years, the number of urban dwellers will swell to an estimated five billion people. With their inefficient transportation systems and poorly designed buildings, many cities consume enormous quantities of fossil fuels and emit high levels of greenhouse gases. But our planet is rapidly running out of the carbon-based fuels that have powered urban growth for centuries and we seem to be unable to curb our greenhouse gas emissions. Are the world's cities headed for inevitable collapse? The authors of this spirited book don't believe that oblivion is necessarily the destiny of urban areas. Instead, they believe that intelligent planning and visionary leadership can help cities meet the impending crises, and look to existing initiatives in cities around the world.