220 million Americans crowd together in the 3% of the country that is urban. 35 million people live in the vast metropolis of Tokyo, the most productive urban area in the world. The central city of Mumbai alone has 12 million people, and Shanghai almost as many. We choose to live cheek by jowl, in a planet with vast amounts of space. Yet despite all of the land available to us, we choose to live in proximity to cities. Using economics to understand this phenomenon, the urban economist uses the tools of economic theory and empirical data to explain why cities exist and to analyze urban issues...