The shift of a population from rural to urban living. More than half the world's people now live in cities, a threshold crossed around 2007, and the transition is among the largest changes in how humans live.
For nearly all of human history the great majority lived rurally and worked in agriculture.
In 1800 around three per cent of the world's population lived in cities. By 1900 it was roughly fifteen per cent. It passed fifty per cent around 2007 and continues to rise, with projections placing it near seventy per cent by 2050.
The absolute numbers are larger still because total population grew at the same time, so the urban population has multiplied many times over.
Agricultural productivity is the precondition. A society in which most people must farm to feed everyone cannot urbanise, and rising yields release labour.
Industrial employment concentrated work in fixed locations, which is why the first sustained urbanisation accompanied industrialisation in Europe and North America.
Economic opportunity drives migration. Cities offer higher wages, more varied employment, and access to services, and the wage gap between urban and rural work is a consistent predictor of migration rates.
Agglomeration effects make cities productive out of proportion to their size. Firms benefit from access to labour, suppliers and customers, and ideas spread faster where people are dense. Measured productivity per person rises with city size across countries, which is among the more robust findings in urban economics.
Natural increase now contributes more than migration in many countries, meaning urban populations grow because urban residents have children rather than because people arrive.
The pattern has shifted geographically. European and North American urbanisation largely completed in the twentieth century, and current growth is concentrated in Asia and Africa.
The pace differs from the historical case. Cities that took a century to reach several million in Europe have been matched in decades elsewhere, which compresses the time available to build infrastructure.
Megacities, conventionally those above ten million, have multiplied, and most are in the developing world. A large share of urban growth nonetheless occurs in small and medium cities rather than the largest.
Informal settlement is a defining feature of rapid urbanisation. Around a quarter of the world's urban population lives in settlements lacking secure tenure or adequate services, and where formal housing supply cannot match arrival rates, informal building fills the gap.
Cities are efficient in resource terms. Per person, dense urban living generally consumes less land, less energy for transport and less material for infrastructure than dispersed settlement, and urban residents in wealthy countries frequently have lower per-capita emissions than rural ones in the same country.
They concentrate services. Hospitals, universities and specialised employment require a population threshold to be viable.
Against this, cities concentrate pollution, and urban air quality is a major cause of illness. They amplify infectious disease transmission, as density increases contact rates. And rapid growth outpaces sanitation, water and transport, which is where the health burden of urbanisation falls.
Urban inequality is generally higher than rural, since cities contain both the greatest opportunity and the least secure housing.
The evidence indicates the outcome depends less on the rate of growth than on whether institutions keep pace.
Land tenure matters. Where residents have secure rights, they invest in their own housing, and upgrading informal settlements in place has generally outperformed clearance and relocation.
Infrastructure ahead of demand is far cheaper than retrofitting. Laying out street grids and reserving corridors before development is inexpensive; acquiring land afterwards is not.
Governance capacity, meaning the ability to plan, tax and deliver services, is the variable most consistently associated with whether rapid urbanisation produces functioning cities or persistent deprivation.
The zoning and public transport capsules describe two of the specific instruments involved.
Urbanisation is the context for most other questions about how people now live: housing cost, transport, air quality, infectious disease and energy use are all shaped by the fact that the majority of humanity has moved into cities within living memory.
It is also close to irreversible in practice. No large country has substantially reversed the process, and the practical question is therefore how cities are built rather than whether the shift continues.
