By the middle of the 19th century, rapid urban growth spurred by the industrial revolution had created urgent social and environmental problems. Cities responded by building centralized networks to deliver clean water, energy and safe food; to enable commerce, facilitate transportation and maintain order; and to provide access to health care and energy. Today these century-plus-old solutions are increasingly inadequate. Many of our cities are jammed with traffic. Our political institutions are deadlocked. In addition, we face a host of new challenges—most notably, feeding and housing a population set to grow by two billion people while simultaneously preventing the worst impacts of global warming.