Urbanization is a population shift from rural to urban areas, "the gradual increase in the proportion of people living in urban areas", and the ways in which each society adapts to the change. It predominantly results in the physical growth of urban areas, be it horizontal or vertical. The United Nations projected that half of the world's population would live in urban areas at the end of 2008. It is predicted that by 2050 about 64% of the developing world and 86% of the developed world will be urbanized.That is equivalent to approximately 3 billion urbanites by 2050, much of which will occur in Africa and Asia. 
Urbanization is relevant to a range of disciplines, including geography, sociology, economics, urban planning, and public health. The phenomenon has been closely linked to modernization, industrialization, and the sociological process of rationalization. Urbanization can be seen as a specific condition at a set time (e.g. the proportion of total population or area in cities or towns) or as an increase in that condition over time. So urbanization can be quantified either in terms of, say, the level of urban development relative to the overall population, or as the rate at which the urban proportion of the population is increasing. Urbanization creates enormous social, economic and environmental changes, which provide an opportunity for sustainability with the “potential to use resources more efficiently, to create more sustainable land use and to protect the biodiversity of natural ecosystems.” 
Urbanization is not merely a modern phenomenon, but a rapid and historic transformation of human social roots on a global scale, whereby predominantly rural culture is being rapidly replaced by predominantly urban culture. The first major change in settlement patterns was the accumulation of hunter-gatherers into villages many thousand years ago. Village culture is characterized by common bloodlines, intimate relationships, and communal behavior whereas urban culture is characterized by distant bloodlines, unfamiliar relations, and competitive behavior. This unprecedented movement of people is forecast to continue and intensify during the next few decades, mushrooming cities to sizes unthinkable only a century ago. Today, in Asia the urban agglomerations of Osaka, Karachi, Jakarta, Mumbai, Shanghai, Manila, Seoul, and Beijing are each already home to over 20 million people, while the Pearl River Delta, Delhi and Tokyo are forecast to approach or exceed 40 million people each within the coming decade. Outside Asia, Mexico City, Sao Paulo, New York City, Lagos, Los Angeles, and Cairo are fast approaching being, or are already, home to over 20 million people.
From the development of the earliest cities in Mesopotamia and Egypt until the 18th century, an equilibrium existed between the vast majority of the population who engaged in subsistence agriculture in a rural context, and small centres of populations in the towns where economic activity consisted primarily of trade at markets and manufactures on a small scale. Due to the primitive and relatively stagnant state of agriculture throughout this period the ratio of rural to urban population remained at a fixed equilibrium.
With the onset of the agricultural and industrial revolution in the late 18th century this relationship was finally broken and an unprecedented growth in urban population took place over the course of the 19th century, both through continued migration from the countryside and due to the tremendous demographic expansion that occurred at that time. In England, the urban population jumped from 17% in 1801 to 72% in 1891 (for other countries the figure was: 37% in France, 41% in Prussia and 28% in the United States).
As labourers were freed up from working the land due to higher agricultural productivity they converged on the new industrial cities like Manchester and Birmingham which were experiencing a boom in commerce, trade and industry. Growing trade around the world also allowed cereals to be imported from North America and refrigerated meat from Australasia and South America. Spatially, cities also expanded due to the development of public transport systems, which facilitated commutes of longer distances to the city centre for the working class.
Urbanization rapidly spread across the Western world and, since the 1950s, it has begun to take hold in the developing world as well. At the turn of the 20th century, just 15% of the world population lived in cities. According to the UN the year 2007 witnessed the turning point when more than 50% of the world population were living in cities, for the first time in human history.
As more and more people leave villages and farms to live in cities, urban growth results. The rapid growth of cities like Chicago in the late 19th century, Tokyo in the mid twentieth, and Delhi in the 21st century can be attributed largely to rural-urban migration. This kind of growth is especially commonplace in developing countries. This phenomenal growth can also be attributed to the lure of not just economic opportunities, but also to loss or degradation of farmland and pastureland due to development, pollution, land grabs, or conflict, the attraction and anonymity of hedonistic pleasures of urban areas, proximity and ease of mass transport, as well as the opportunity to assert individualism.
Urban centres are seen by many as an opportunity to “escape traditional patriarchy and experience new freedoms,” this includes greater access to education, health, and employment. However, for many who seek these opportunities the opposite occurs resulting in “extreme poverty, exclusion, vulnerability and marginalization due to urban sprawl where “urban land is expanding much faster than the urban population.”  As such a strain on urban efficiencies occurs resulting in the urban poor who are forced to create slums and then ultimately face unhealthy living conditions without access to the very opportunities they sought in the first place. The United Nations Population Fund (UNFPA) estimated that residents in slums had risen to approximately 863 million in 2012 from over 650 million in 1990. 
The rapid urbanization of the world’s population over the twentieth century is described in the 2005 Revision of the UN World Urbanization Prospects report. The global proportion of urban population rose dramatically from 13% (220 million) in 1900, to 29% (732 million) in 1950, to 49% (3.2 billion) in 2005. The same report projected that the figure is likely to rise to 60% (4.9 billion) by 2030.
According to the UNFPA State of the World Population 2007 report, sometime in the middle of 2007, the majority of people worldwide will be living in towns or cities, for the first time in history; this is referred to as the arrival of the "Urban Millennium" or the 'tipping point'. In regard to future trends, it is estimated 93% of urban growth will occur in developing nations, with 80% of urban growth occurring in Asia and Africa.
Urbanization rates vary between countries. The United States and United Kingdom have a far higher urbanization level than India, Swaziland or Niger, but a far slower annual urbanization rate, since much less of the population is living in a rural area. Some nations make a distinction between suburban and urban areas, while others do not; indeed, human conditions within such areas differ greatly.
- Urbanization in the United States never reached the Rocky Mountains in locations such as Jackson Hole, Wyoming; Telluride, Colorado; Taos, New Mexico; Douglas County, Colorado and Aspen, Colorado. The state of Vermont has also been affected, as has the coast of Florida, the Birmingham-Jefferson County, AL area, the Pacific Northwest and the barrier islands of North Carolina.
- In the United Kingdom, two major examples of new urbanization can be seen in Swindon, Wiltshire and Milton Keynes, Buckinghamshire. These two towns show some of the quickest growth rates in Europe.
Urbanization occurs as individual, commercial flight, social and governmental efforts reduce time and expense in commuting and transportation and improve opportunities for jobs, education, housing, and transportation. Living in cities permits the advantages of the opportunities of proximity, diversity, and marketplace competition. However, the advantages of urbanization are weighed against alienation issues, stress, increased daily life costs, and negative social aspects that result from mass marginalization. Suburbanization, which is happening in the cities of the largest developing countries, was sold and seen as an attempt to balance these negative aspects of urban life while still allowing access to the large extent of shared resources.
Cities are known to be places where money, services, wealth and opportunities are centralized. Many rural inhabitants come to the city for reasons of seeking fortunes and social mobility. Businesses, which provide jobs and exchange capital are more concentrated in urban areas. Whether the source is trade or tourism, it is also through the ports or banking systems that foreign money flows into a country, commonly located in cities.
Economic opportunities are just one reason people move into cities, though they do not go to fully explain why urbanization rates have exploded only recently in places like China and India. Rural flight is a contributing factor to urbanization. In rural areas, often on small family farms or collective farms in villages, it has traditionally been difficult to access manufactured goods, though overall quality of life is very subjective, and may certainly surpass that of the city. Farm living has always been susceptible to unpredictable environmental conditions, and in times of drought, flood or pestilence, survival may become extremely problematic.
In a New York Times article concerning the acute migration away from farming in Thailand, life as a farmer was described as "hot and exhausting." "Everyone says the farmer works the hardest but gets the least amount of money". In an effort to counter this impression, the Agriculture Department of Thailand is seeking to promote the impression that farming is "honorable and secure".
However, in Thailand, urbanization has also resulted in massive increases in problems such as obesity. City life, especially in modern urban slums of the developing world, is certainly hardly immune to pestilence or climatic disturbances such as floods, yet continues to strongly attract migrants. Examples of this were the 2011 Thailand floods and 2007 Jakarta flood. Urban areas are also far more prone to violence, drugs, and other urban social problems. In the case of the United States, industrialization of agriculture has negatively affected the economy of small and middle-sized farms and strongly reduced the size of the rural labour market.
Particularly in the developing world, conflict over land rights due to the effects of globalization has led to less politically powerful groups, such as farmers, losing or forfeiting their land, resulting in obligatory migration into cities. In China, where land acquisition measures are forceful, there has been far more extensive and rapid urbanization (54%) than in India (36%), where peasants form militant groups (e.g. Naxalites) to oppose such efforts. Obligatory and unplanned migration often results in rapid growth of slums. This is also similar to areas of violent conflict, where people are driven off their land due to violence. Bogota, Colombia is one example of this.
Cities offer a larger variety of services, such as specialist services that aren't found in rural areas. Supporting the provision of these services requires workers, resulting in more numerous and varied job opportunities. Elderly individuals may be forced to move to cities where there are doctors and hospitals that can cater for their health needs. Varied and high quality educational opportunities are another factor in urban migration, as well as the opportunity to join, develop, and seek out social communities.
Urbanization also creates greater opportunities for women that are otherwise denied to them living in rural areas. This creates a gender-related transformation where women are engaged in paid employment and have access to education resulting in demographic implications in which fertility levels decline. However, women are still at a disadvantage due to their unequal position in the labour market, their inability to secure assets independently from male relatives and exposure to violence. 
People located in cities are more productive than those working outside dense agglomerations. An important question for the policy makers as well as for clustering people deals with the causality of this relationship, that is whether people become more productive in cities due to certain agglomeration effects or are cities simply attracting those who are more productive. Economists have recently shown that there exists indeed a large productivity gain due to locating in dense agglomerations. It is thus possible that agents locate in cities in order to benefit from these agglomeration effects.
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The dominant conurbation(s) of a country also benefit from even more intense concentrations of the very same things cities offer, making them magnets for not just the non-urban population, but urban and suburban population from other conurbations. Dominant conurbations are quite often primate cities, but do not have to be. Due to cases like Greater Manila, conurbation rather than city is more apt; as a whole Greater Manila's 20 million (over 20% national population) is very much a primate city, yet Quezon City(2.7 million), the largest municipality, or Manila (1.6 million), the capital, is not. Measures of a how dominant a conurbation is can relate to percentage of national output, wealth, and especially population as a percentage of an entire country. Greater Seoul is one conurbation with massive dominance over South Korea, it is home to 50% of the entire national population.
Though Greater Busan-Ulsan (15%, 8 million) and Greater Osaka (14%, 18 million) exhibit strong dominance in their respective countries, yet they are losing population to even more dominant rivals, Seoul and Tokyo.
As cities develop, effects can include a dramatic increase and change in costs, often pricing the local working class out of the market, including such functionaries as employees of the local municipalities. For example, Eric Hobsbawm's book The age of revolution: 1789–1848 (published 1962 and 2005) chapter 11, stated "Urban development in our period [1789–1848] was a gigantic process of class segregation, which pushed the new labouring poor into great morasses of misery outside the centres of government and business and the newly specialized residential areas of the bourgeoisie. The almost universal European division into a 'good' west end and a 'poor' east end of large cities developed in this period." This is likely due the prevailing south-west wind which carries coal smoke and other airborne pollutants downwind, making the western edges of towns preferable to the eastern ones. Similar problems now affect the developing world, rising inequality resulting from rapid urbanization trends. The drive for rapid urban growth and often efficiency can lead to less equitable urban development. Think tanks such as the Overseas Development Institute have proposed policies that encourage labor-intensive growth as a means of absorbing the influx of low-skilled and unskilled labor. One problem these migrant workers are involved with is the growth of slums. In many cases, the rural-urban low skilled or unskilled migrant workers, attracted by economic opportunities in urban areas, cannot find a job and afford housing in cities and have to dwell in slums. Urban problems, along with infrastructure developments, are also fueling suburbanization trends in developing nations, though the trend for core cities in said nations tends to continue to become ever denser. Urbanization is often viewed as a negative trend, but there are positives in the reduction of expenses in commuting and transportation while improving opportunities for jobs, education, housing, and transportation. Living in cities permits individuals and families to take advantage of the opportunities of proximity and diversity. While cities have a greater variety of markets and goods than rural areas, infrastructure congestion, monopolization, high overhead costs, and the inconvenience of cross-town trips frequently combine to make marketplace competition harsher in cities than in rural areas.
In many developing countries where economies are growing, the growth is often erratic and based on a small number of industries. For young people in these countries barriers exist such as, lack of access to financial services and business advisory services, difficulty in obtaining credit to start a business, and lack of entrepreneurial skills, in order for them to access opportunities in these industries. Investment in human capital so that young people have access to quality education and infrastructure to enable access to educational facilities is imperative to overcoming economic barriers. 
The phenomenon of Urban heat islands has become a growing concern. Incidence of this phenomenon as well as concern about it has increased over the years. An urban heat island is formed when industrial and urban areas are developed resulting in greater production and retention of heat. A large proportion of solar energy that affects rural areas is consumed by the evaporating water from vegetation and soil. In cities, where there is less vegetation and exposed soil, the majority of the sun’s energy is absorbed by urban structures and asphalt. Hence, during warm daylight hours, less evaporative cooling in cities results in higher surface temperatures than in rural areas. Vehicles and factories release additional city heat, as do industrial and domestic heating and cooling units. As a result, cities are often 1.8 to 5.4 °F (1 to 3 °C) warmer than surrounding landscapes. Impacts also include reducing soil moisture and a reduction in re-uptake of carbon dioxide emissions.
In his book Whole Earth Discipline, Stewart Brand argues that the effects of urbanization are primarily positive for the environment. Firstly, the birth rate of new urban dwellers falls immediately to replacement rate, and keeps falling, reducing the risk of environmental stresses caused by population growth. Secondly, migration away from rural areas reduces the prevalence of destructive subsistence farming techniques, such as improperly implemented slash and burn agriculture.
In July 2013 a report was issued by the United Nations Department of Economic and Social Affairs, however warns that with the additional 2.4 billion people by 2050, the amount of food produced will have to increase by 70 percent straining food resources, especially in countries already facing food insecurity due to changing environmental conditions. The mix of changing environmental conditions and the growing number of people living in urban regions, according to UN experts, will strain basic sanitation systems, health care, and potentially cause a humanitarian and environmental nightmare.
In the developing world, urbanization does not seem to translate into a significant increase in life expectancy. Rapid urbanization has brought increased mortality from non-communicable diseases associated with lifestyle, including cancer and heart disease. Differences in mortality from contagious diseases vary depending on the particular disease.
Urban health levels are better in comparison those in rural areas on average. However, residents in poor areas such as slums and informal settlements suffer “disproportionately from disease, injury, premature death, and the combination of ill-health and poverty entrenches disadvantage over time.”  Financial constraints for many of the urban poor also inhibit their access to health services due to an increasing requirement to pay for services resulting in people resorting to seeing less qualified and unregulated providers.
While urbanization is associated with improvements in public hygiene, sanitation and access to health care, it also entails changes in occupational, dietary and exercise patterns. It can have mixed effects on health patterns, alleviating some problems and accentuating others. For instance, in children urbanization is associated with a lower risk of undernutrition but a higher risk of overweight. Overall, body mass index and cholesterol levels increase sharply with national income and the degree of urbanization. Easier access to non-traditional foods may lead to less healthy dietary patterns. In India prevalence of diabetes in urban areas appears to be more than twice as high as in rural areas. In general, major risk factors for chronic diseases are more prevalent in urban environments.
Different forms of urbanization can be classified depending on the style of architecture and planning methods as well as historic growth of areas.
In cities of the developed world urbanization traditionally exhibited a concentration of human activities and settlements around the downtown area, the so-called in-migration. In-migration refers to migration from former colonies and similar places. The fact that many immigrants settle in impoverished city centres led to the notion of the "peripheralization of the core", which simply describes that people who used to be at the periphery of the former empires now live right in the centre.
Recent developments, such as inner-city redevelopment schemes, mean that new arrivals in cities no longer necessarily settle in the centre. In some developed regions, the reverse effect, originally called counter urbanization has occurred, with cities losing population to rural areas, and is particularly common for richer families. This has been possible because of improved communications, and has been caused by factors such as the fear of crime and poor urban environments. It has contributed to the phenomenon of shrinking cities experienced by some parts of the industrialized world.
When the residential area shifts outward, this is called suburbanization. A number of researchers and writers suggest that suburbanization has gone so far to form new points of concentration outside the downtown both in developed and developing countries such as India. This networked, poly-centric form of concentration is considered by some emerging pattern of urbanization. It is called variously exurbia, edge city (Garreau, 1991), network city (Batten, 1995), or postmodern city (Dear, 2000). Los Angeles is the best-known example of this type of urbanization. Interestingly, in the United States, this process has reversed as of 2011, with "re-urbanization" occurring as suburban flight due to chronically high transport costs.
Rural migrants are attracted by the possibilities that cities can offer, but often settle in shanty towns and experience extreme poverty. The inability of countries to provide adequate housing for these rural migrants is related to overurbanization, a phenomenon in which the rate of urbanization grows more rapidly that the rate of economic development, leading to high unemployment and high demand for resources. In the 1980s, this was attempted to be tackled with the urban bias theory which was promoted by Michael Lipton.
Most of the urban poor in developing countries able to find work can spend their lives in insecure, poorly paid jobs. According to research by the Overseas Development Institute pro-poor urbanization will require labour-intensive growth, supported by labour protection, flexible land use regulation and investments in basic services.'
Urbanization can be planned urbanization or organic. Planned urbanization, i.e.: planned community or the garden city movement, is based on an advance plan, which can be prepared for military, aesthetic, economic or urban design reasons. Examples can be seen in many ancient cities; although with exploration came the collision of nations, which meant that many invaded cities took on the desired planned characteristics of their occupiers. Many ancient organic cities experienced redevelopment for military and economic purposes, new roads carved through the cities, and new parcels of land were cordoned off serving various planned purposes giving cities distinctive geometric designs. UN agencies prefer to see urban infrastructure installed before urbanization occurs. Landscape planners are responsible for landscape infrastructure (public parks, sustainable urban drainage systems, greenways etc.) which can be planned before urbanization takes place, or afterward to revitalize an area and create greater livability within a region. Concepts of control of the urban expansion are considered in the American Institute of Planners.
- Back to the land
- Counter urbanization
- Division of labour
- Megalopolis (city type)
- People Flow
- Political demography
- Suburban sprawl
- Urban ecology
- Urban metabolism
- Urban morphology
- Urban studies
- White flight
Contributors to urbanization:
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The process whereby a society changes from a rural to an urban way of life. It refers also to the gradual increase in the proportion of people living in urban areas.
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|Wikimedia Commons has media related to Urbanization.|
- World Urbanization Prospects, the 2014 Revision, Website of the United Nations Population Division
- NASA Night Satellite Imagery – City lights can provide a simple, visual measure of urbanization
- Geopolis: research group, University of Paris-Diderot, France
- Tomorrow's Crises Today – the humanitarian dimension of urbanization, by IRIN
- The Natural History of Urbanization, by Lewis Mumford
- The World System urbanization dynamics, by Andrey Korotayev
- Brief review of world socio-demographic trends includes review of global urbanization trends
- World Economic and Social Survey 2013, United Nations Department of Economic and Social Affairs.