First, these collected papers stress the heterogeneity of the contemporary city; so, not only the difference between metropolis in the North and in the South of the world, but also between European and American cities. Second, the connection between sociological analyses and urban planning is considered. There are two approaches in urban sociology, sometimes synergic and sometimes conflicting: the critical approach and the pragmatic approach. Today, criticism and pragmatism are both part of many studies of similar urban topics. The authors of these papers are sociologists, architects, and urban planners from the School of Architecture of M.I.T. (Massachusetts Institute of Technology, Boston) and from the Faculty of Architecture of University of Florence. They have different approaches and research methods; however, they agree in stressing the connection between sociology and city planning. These papers differ on their approach, topics, and cultural background. Nevertheless, all of them underline the need to know how to govern and plan the city; it is getting more and more complicated, but necessary and fascinating as well.