Crime, congestion and pollution mar all cities, from Los Angeles to Mumbai. But another force trumps the drawbacks of urban living: cities bring opportunities for wealth and for the creative inspiration that can result only from face-to-face contact with others. In fact, the crush of people living in close quarters fosters the kind of collaborative creativity that has produced some of humanity’s best ideas, including the industrial revolution and the digital age. In the years ahead such collaborations can be expected to help solve the world’s most pressing problems—poverty, energy shortages, climate change—and to promote the type of fundamental political transitions seen in Cairo that recently astonished the world.