It’s hard to pin down the precise moment the world’s center of gravity shifted. For thousands of years, people lived in the countryside. They worked on farms or in villages, knew little of the world beyond their immediate families and neigh­bors, and generally got by on their own. Slowly, they began to congregate. It happened in Mesopotamia and Egypt, later in Greece and Rome, and also in Europe and the Americas. More recently, we’ve seen fast growth in Africa and, most spectacularly, in Asia. And then, by 2008, according to the United ­Nations, the balance finally tipped: in the ebb and flow of daily births and deaths, the number of people who inhabit the world’s cities ticked into the majority, for the first time ever.

The milestone itself isn’t nearly as significant as the trend. In the 20th century cities grew more than 10-fold, from 250 million people to 2.8 billion. In the coming decades, the U.N. predicts, the number of people living in cities will continue to rise. By 2050 the world population is expected to surpass nine billion and urban dwellers to surpass six billion. Two in three people born in the next 30 years will live in cities.

Many otherwise lucid thinkers, from Tho­mas Jefferson to Frank Lloyd Wright to President Gerald Ford, tended to think of cities as centers of poverty, crime, pollution, con­gestion and poor health. In recent years, though, the thinking has shifted along with the demographics. Many experts have come to realize that people are better off when they live in a city. This is not to dismiss the problems of urban life; cities, particularly fast-growing ones in the poorer parts of Asia and Africa, can be places of great human suffering. But even a city slum has benefits that you won’t find on the farm or in the village. The move from the country leads, for instance, to dramatic changes for many women. As Kavita N. Ramdas of the Global Fund for Women notes in Stewart Brand’s Whole Earth Discipline (Penguin, 2010), “In the village, all there is for a woman is to obey her husband and relatives, pound millet, and sing. If she moves to town, she can get a job, start a business, and get education for her children.”

Indeed, the city has come to look less like a source of problems than as an opportunity to fix them. Investments in sanitation and water have turned many cities in the developed world from places of disease and pestilence into bastions of health. City folk are at lower risk of death from motor vehicle accidents and suicide by firearms (although they are overstressed). From the stand­point of the metropolis, climate change also seems less intractable. Because city residents rely less on cars and live in more compact dwellings than suburbanites, they tend to leave smaller carbon footprints. The challenge is to extend the efficiency of the urban center to the wider conurbation, embracing the city center, suburbs and satellite towns. Although climate is bigger than any one fix, how we build our cities, and how efficiently we live in them, is going to factor large in our response.

The most hopeful impact of city life may be its effect on the mind. Humans are social animals; we draw stimulation from other minds close at hand. Plato and Socrates both lived in fifth-century b.c. Athens, a city-state. Galileo and Michelangelo lived in Renaissance Florence. Steve Jobs and Steve Wozniak grew up in a western U.S. conurbation that includes Silicon Valley. The young, agile minds at work on the next Big Thing are probably tweeting—they live, as author William Gibson points out on page 88, in a kind of digital meta city. Chances are, they are living in a physical city, too. Technology is reshaping city life and making it more intellectually productive, but it will not soon replace the easy interchange of ideas that comes from casual proximity, the cornerstone of city life.

This issue of Scientific American celebrates the city as a solution to the problems of our age. We have tried to present it in the true urban spirit: best ideas forward.