The twenty-first century is the first urban century in human history, the first time more people on the planet live in cities than don’t. Experts project that some 75 percent of the booming global population will be city dwellers by 2050. Dozens of new cities are springing up in Asia, their growth hastened by political unrest, climate change, and mass relocation programs that have cleared vast swaths of the Chinese countryside. Much of the growth in countries like India and Bangladesh is chaotic and badly planned. In many growing cities across the Global South there are serious shortages of water, sanitation, and housing, along with increasing air pollution. The United States has some of the same problems on a smaller scale, while here urban development is also being stimulated by growing numbers of university graduates and empty-nesters who are rejuvenating downtowns and rejecting suburbia, the culture of commuting, sprawl, and the automobile.

Not that suburbs have stopped growing, but since the late 1990s, the share of automobiles driven by people in their twenties in America has fallen from 20.8 percent to 13.7 percent. The number of nineteen-year-olds opting out of driver’s licenses has tripled since the 1970s from 8 to 23 percent. Electric, self-driving vehicles may soon revolutionize transportation and urban land use. Meanwhile, deindustrialization, plummeting crime rates, and increasing populations of singles and complex, nontraditional families have reshaped many formerly desolate urban neighborhoods.

People are moving downtown for jobs, but also for the pleasures and benefits of cultural exchange, walkable streets, parks, and public squares. Squares have defined urban living since the dawn of democracy, from which they are inseparable. The public square has always been synonymous with a society that acknowledges public life and a life in public, which is to say a society distinguishing the individual from the state. There were, strictly speaking, no public squares in ancient Egypt or India or Mesopotamia. There were courts outside temples and royal houses, and some wide processional streets.

By the sixth century BC, the agora in Athens was a civic center, and with the rise of democracy, became a center for democracy’s institutions, the heart of public life. In ancient Greek, the word “agora” is hard to translate. In Homer it could imply a “gathering” or “assembly”; by the time of Thucydides it had come to connote the public center of a city, the place around which the rest of the city was arranged, where business and politics were conducted in public—the place without which Greeks did not really regard a town or city as a town or city at all. Rather, such a place was, as the second-century writer Pausanias roughly put it, just a sorry assortment of houses and ancient shrines.

The agora announced the town as a polis. Agoras grew in significance during the Classical and…