URBANIZATION IS THE process by which large numbers of people become permanently concentrated in relatively small areas, forming cities. The definition of what constitutes a city changes from time to time and place to place, but it is most usual to explain the term as a matter of demographics. The United Nations has recommended that countries regard all places with more than 20,000 inhabitants living close together as urban; but, in fact, nations compile their statistics on the basis of many different standards. For instance, the UNITED STATES uses “urban place” to mean any locality where more than 2,500 people live.

Large permanent assemblies of people have arisen historically in two sharply contrasting ways. Some settlements emerge because a group of people chose to live near each other in order to realize a way life made possible by production activities carried on elsewhere. Other settlements arise because people who work within certain production facilities wish to live in the immediate vicinity of these facilities. In general, then, population may be permanently assembled in order to consume the products and services of labor, regardless of where they are produced, or in order to produce products and services, regardless of where they are consumed.


The words produce and consume are used in their broadest senses, including not only traditional economic activities, such as commerce and manufacture, but also religious, military, professional, educational, and other similarly organized activities. Examples of consumption-oriented settlements include the modern residential suburbs, the traditional rural village, the ceremonial capital or “court” city, and the urban community of the elite during the social season. Examples of the production-oriented settlements include the manufacturing city, the market town, and the government administrative center, ranging from county seat to national capital.

Whatever the numerical definition, it is clear that the course of human history has been marked by a process of accelerated urbanization. It was not until the Neolithic period, roughly 10,000 years ago, that humans were able to form permanent settlements. Even 5,000 years ago, the only such settlements on the globe were small, semipermanent villages of peasant farmers, towns whose size was limited by the fact that people had to move whenever the soil nearby was exhausted. It was not until the time of classical antiquity that cities of more than 100,000 existed, and even these did not become common until the sustained population explosion of the last three centuries. In 1800, less than 3 percent of the world's population was living in cities of 20,000 or more; this increased to about 25 percent by the mid-1960s and to about 40 percent by 1980. It is estimated that now more than half of the world's population lives in urban areas.

The little towns of ancient civilizations, in both the old world and the new, were only possible because of improvements in agriculture and transportation. As farming became more productive, it produced a surplus food. The development of means of transportation, dating from the invention of the wheel in about 3500 B.C.E., made it possible for the surplus from the countryside to feed urban populations, a system that continues to the present day.

Despite the small size of these villages, the people in the early towns lived quite close together. Distances could be no longer than an easy walk, and nobody could live out of the range of the water supply. In addition, because cities were constantly subject to attack, they were quite often walled, and it was difficult to extend barricades over a large area. Archaeological excavations have suggested that the population density in the cities of 2000 B.C.E. may have been as much as 128,000 per square mi (49,400 per square km); today KOLKATA (formerly Calcutta) and SHANGHAI, with densities of more than 70,000 per square mi (112,654 per square km), are regarded as extremes of overcrowding.

With few exceptions, the elite—aristocrats, government officials, clergy, and the wealthy—lived in the center of ancient cities, which was usually located near the most important temple. Farther out were the poor, who sometimes huddled along the city walls together. However, the situation reversed in the 20th century, when most cities became surrounded by rings of rich suburbs, and only the poor were left in the city centers. In the United States, the suburbs are populated by the affluent and the middle class, who grew up around cities in the 1950s and 1960s, abandoning inner cities.

The greatest city of antiquity was Rome, which at its zenith in the 3rd century covered almost 4 square mi (10 square km) and had at least 800,000 inhabitants. To support this enormous population, the empire constructed a system of aqueducts that channeled drinking water from hills as far away as 44 mi (70 km). Inside the city itself, the water was pumped to individual homes through a remarkable network of conduits and lead pipes, the equal of which was not seen until the 20th century. As in most early cities, Roman housing was initially built on dried clay molded by wooden frameworks. As the city grew, it began to include structures made from mud, brick concrete, and eventually finely carved marble.

This general model of city structure continued until the advent of the INDUSTRIAL REVOLUTION, although medieval towns were rarely as large as Rome. Over the course of time, commerce became an increasingly important part of city life and drew people from the countryside. With the invention of the mechanical clock, windmill, water mill, and the printing press, the interconnection of city inhabitants continued apace. Cities became places where all classes and type of humanity mingled, creating heterogeneity that became one of the most celebrated features of urban life.

The technological explosion that was the Industrial Revolution led to a momentous increase in the process of urbanization. Larger populations in small areas meant that the new factories could draw on a big pool of workers in Europe, many of them living in miserable conditions attracted by a promise of paid work. Migrants from the rural areas flooded into the city, only to find it awash in refuse, disease, and rodents. Designed for commerce, the streets of the newer cities were often arranged in grid patterns that took little account of human needs, such as privacy and recreation, but allowed these cities to expand indefinitely. Although highly urbanized areas are generally highly industrialized areas, urbanization is not a simple function of industrialization.

It is speculated that one result of the continuing population explosion will be the creation of megalopolises, concentrations of urban centers that may extend scores of miles. It is thought that the first such growth could occur on the East Coast of the United States, where there may eventually be a single urban agglomeration stretching from Boston to Washington, D.C. Other emerging megalopolises include the Tokyo-Osaka-Kyoto complex in JAPAN, the region between LONDON and the Midland cities in Great Britain, and the NETHERLANDS-central BELGIUM area.