Urbanization is the process through which an increasing proportion of a population lives in urban settlements, including cities and towns. In demography, it refers primarily to a change in population distribution rather than simply an increase in the number of urban residents. More broadly, it encompasses transformations in employment, land use, settlement patterns, and the provision of infrastructure. Urbanization and urban population growth are related but distinct: an urban population can grow without its share of the total population increasing. (population.un.org)
Definitions and measurement
The level of urbanization is commonly expressed as the urban population divided by the total population, multiplied by 100. Its measurement depends on what counts as “urban.” National definitions may use administrative status, population thresholds, density, economic characteristics, or combinations of these criteria. Consequently, figures from different countries are not always directly comparable, and boundary changes can affect recorded trends. (population.un.org)
The internationally harmonized Degree of Urbanisation classification distinguishes cities, towns and semi-dense areas, and rural areas using population size, density, and spatial contiguity. It complements rather than replaces national definitions. A functional urban area, by contrast, includes a city and its economically connected commuting zone; it measures relationships extending beyond the densely built-up core. These approaches answer different questions about settlement concentration and economic integration. (oecd.org)
Definition changes are especially important when interpreting global figures. The United Nations’ World Urbanization Prospects 2025 estimated that cities contained 45 percent of the world’s 8.2 billion inhabitants in 2025 under its harmonized settlement classification. This is a city-only figure, not the urban share obtained by combining cities with towns or applying national definitions. The report provides estimates for 237 countries or areas and projections through 2050. (un.org)
Historical development
Urban concentration long predates industrial society. In Mesopotamia, Uruk had become a major urban settlement by approximately 3200 BCE. Monumental buildings, specialized production, administrative records, and the organization of goods and workers demonstrate the complexity of early urban life. Such developments show that cities could emerge through institutional and commercial concentration without modern industrial technology. (metmuseum.org)
The Industrial Revolution greatly accelerated urban population growth from the late eighteenth century onward. Expanding manufacturing drew workers into industrial settlements, while demographic growth enlarged their populations. During the twentieth and twenty-first centuries, urbanization became increasingly global, but countries followed different trajectories rather than a single sequence of industrialization and city growth. (en.wikipedia.org)
Urbanization does not mean that every city continually expands. Some cities experience population aging or decline, while others grow rapidly. The UN’s 2025 assessment also emphasizes that most city residents live in small and medium-sized cities rather than megacities; many smaller cities in Africa and Asia are among the fastest-growing settlements. (worldbank.org)
Population processes and economic drivers
Urban population change has three principal components: natural increase, net migration, and settlement reclassification. Natural increase occurs when births exceed deaths. Migration adds population when arrivals exceed departures, including movement from rural areas and other countries. Reclassification occurs when settlements cross urban thresholds or authorities incorporate formerly rural territory into urban boundaries. It can increase the recorded urban population without anyone changing residence. (un.org)
Migration reflects both opportunities at destinations and pressures in places of origin. Urban employment, services, and prospects for advancement attract migrants, while poverty, conflict, and climate-related events may also influence movement. The relative importance of these factors varies across countries and periods. (worldbank.org)
Cities can support agglomeration economies by concentrating businesses, workers, and customers. Larger labor markets, shared facilities, and exchanges of knowledge can raise productivity and encourage innovation. These mechanisms connect urbanization with economic growth, but the benefits are conditional: disconnected housing, employment, and transport networks can prevent urban concentration from producing corresponding economic gains. (worldbank.org)
Spatial and social transformations
Urban development occurs through outward expansion, construction on vacant or underused land within existing settlements, and upward growth through taller buildings. Different forms can coexist within one metropolitan area. Thus, population concentration and physical land expansion need not proceed at the same rate. Outward development may consume agricultural land even where population density is falling. (ipcc.ch)
Rapid growth can place pressure on housing, transport, sanitation, and public services. Where affordable housing and serviced land are insufficient, informal settlements may expand. Differences in access to employment and services can also produce social exclusion. These conditions depend not only on population growth but on land institutions, investment, and the capacity to connect neighborhoods with wider urban opportunities. (worldbank.org)
Environmental effects and governance
Urban expansion can cause habitat fragmentation and loss of biodiversity. The urban heat island effect alters local thermal conditions, while settlement growth into hazard-prone locations can increase exposure to flooding and other climate risks. Impacts depend on urban form, infrastructure, and socioeconomic vulnerability as well as climate change. (ipcc.ch)
Compact, connected development can reduce travel distances and support lower greenhouse gas emissions compared with dispersed, automobile-dependent development. Urban planning and urban governance shape these outcomes through land-use decisions, transport investment, housing provision, and municipal finance. Local authorities’ institutional and financial capacities influence whether infrastructure and services keep pace with expanding settlements. (ipcc.ch)