For nearly all of history most people lived in the countryside. In 1800 only a small fraction of humanity, by most estimates well under a tenth, lived in towns or cities. Urbanization is the growth in the share of people who live in urban areas, and it is one of the defining changes of the modern world. The United Nations estimates that the urban share passed half of humanity around 2007, and it projects roughly two thirds by 2050.

Why cities grow

Cities grow in two ways: by natural increase, when births exceed deaths, and by migration from the countryside or from abroad. The deepest attraction of cities is economic. Agglomeration economies are the benefits firms and workers gain from being close together: larger pools of workers and customers, shared suppliers and infrastructure, and the swift spread of ideas when people with similar skills keep running into each other. This is why technology firms cluster in a handful of metropolitan areas, even though their products travel instantly over the internet.

Systems of cities

Cities form networks. In 1933 the German geographer Walter Christaller published central place theory, which explains how settlements of different sizes are spaced across a landscape. Central place theory holds that larger places offer rarer goods and services to wider surrounding areas, while small towns offer everyday goods to the people close by. You can buy bread in a village, but for a heart surgeon or an opera you must travel to a large city. Christaller predicted a neat pattern of hexagonal market areas on a flat plain. Real landscapes are messier, but the idea of a hierarchy still guides planning.

In many countries one city towers over the rest. A primate city is one that dwarfs and dominates all other cities in its country, in its economy, culture and politics. The term was coined by the American geographer Mark Jefferson in 1939. Bangkok, Lima and Paris are common examples. A metropolitan area with more than ten million people is called a megacity; there are now several dozen, most of them in Asia.

Models of the city

Geographers and sociologists have drawn simple models of how land is used inside cities. The concentric zone model of Ernest Burgess, published in 1925 and based on Chicago, pictured rings spreading out from a central business district, with poorer people near the center and richer people in the outer suburbs. Homer Hoyt's sector model of 1939 noted that cities often grow in wedges along transport lines, and the multiple nuclei model of Chauncy Harris and Edward Ullman, published in 1945, described cities with several centers. None of them fits every city. In much of Latin America, for example, wealth has traditionally clustered near the old central plaza and the poor have settled on the edge, nearly the reverse of Burgess's Chicago.

Car-based growth produced suburban sprawl, low-density building that spreads outward and makes walking and public transport difficult. Edge cities are clusters of offices and shops that grow on the outskirts, far from the old downtown, a term popularized by the journalist Joel Garreau in 1991. At the same time many older central neighborhoods have gone through gentrification, as wealthier newcomers move in, renovate and raise prices. Supporters point to new investment and safer streets; critics point to rising rents that push out longtime residents. How a city handles them tests whether it treats its poorest residents as people or as obstacles.

Informal cities

In much of the world, cities have grown faster than governments can build housing, roads, water pipes and sewers. Informal settlements are neighborhoods built without legal title or official planning, often by the residents themselves. UN-Habitat estimates that around a billion people live in slums or informal settlements. They are often portrayed only as places of misery, but they are also places of enterprise, community and steady self-improvement. Policies have ranged from forced clearance, which destroys homes and livelihoods, to upgrading, which brings water, drains, paths, lighting and legal title to existing neighborhoods. Experience has generally favored upgrading where it is possible.

Worked example: Dharavi

Dharavi, in the middle of Mumbai, is often called one of Asia's largest informal settlements. Estimates of its population range from about 600,000 to a million people, packed into roughly two square kilometers. It began on marshland at the edge of the colonial city, but Mumbai grew around it, so its relative location changed: land once on the fringe now sits near the heart of one of the most expensive property markets on Earth. Dharavi is also an economic engine, with thousands of small workshops in leather, pottery, garments and recycling.

Its story brings together the ideas of this level. Agglomeration explains why so much small industry crowds into it. Its central location explains why developers covet it. And the long debate over redevelopment plans, which promise new flats but threaten workshops, livelihoods and community ties, shows the choice between clearance and upgrading playing out in real time. Residents have repeatedly asked a simple question: will the new city have room for the people who built the old one?