Throughout history, people have migrated to cities for a variety of reasons, and the resulting urban population growth has not always been steady. Sometimes, people are pulled toward urban areas. For example, during the Industrial Revolution, factories created job opportunities that drew people away from rural areas. At other times, people are driven out of rural areas by natural disasters, like droughts or flooding. People are also pushed out of rural areas due to violence, such as during one of the Great Migrations, when Black Americans moved to northern U.S. cities to escape discrimination and violence.

Wide-scale urbanization is a relatively recent phenomenon. Until recent years, the vast majority of people lived in rural areas. For instance, in 1800, more than 90% of the global population lived in rural areas. The U.S. Census Bureau indicates that, in 1800, more than 94% of the U.S. population lived in rural areas; by 1900, this number had dropped to 60%.

Still, the first cities appeared thousands of years ago in areas where the land was fertile, such as the cities founded in Mesopotamia (modern-day Iraq) around 7500 B.C.E., which included Eridu, Uruk and Ur. These cities were among the many communities between the Euphrates and Tigris rivers (the so-called Fertile Crescent). Cities also formed along the Nile River in Egypt and Sudan, the Indus River Valley on the Indian subcontinent, and the Huang He (or Yellow River) in China, as people began to cultivate crops and settle in communities. Agricultural production in these fertile areas meant that people could give up their nomadic lifestyle as hunters and gatherers to take advantage of food surpluses. Settling along waterways also provided a much-needed transportation system that facilitated trade.

For the next several millennia, cities continued to grow in number, size and stature. Some of the world’s largest cities have grown steadily for hundreds of years. During the Tokugawa period, which began in 1603, Tokyo became the capital city of Japan. It served as the center of the military government, attracting more residents and establishing itself as a significant city on the global stage.

Some ancient cities that were once among the most populous in the world, such as the cities of Mesopotamia, no longer exist. Others have experienced a decline in population. For instance, in the first century B.C.E., Rome (now in the capital of Italy) topped one million people, making it the largest city in Europe, but during the Middle Ages, its population declined to just 20,000. Rome deteriorated due to government corruption, a poor economy and agriculture, and invasions by foreign forces. Other cities have gone through similar cycles of increasing prominence, innovation and artistry, followed by regression and instability. Baghdad, today the capital of Iraq, was a major center of human civilization in the eighth century, bringing together new scientific ideas, worldwide commerce and advancements in the arts, but its influence faded after the Mongols invaded in 1258, destroying much of the city and its great institutions, including the House of Wisdom. Istanbul, now a city in Türkiye, also has a history of rising and falling population and influence. It was once the capital of two prominent empires, the Eastern Roman Empire and the Ottoman Empire, but it has since experienced periods of significant population decrease due to disease and war.

One of the main reasons for the growth of cities in the modern era was the Industrial Revolution, which began in England toward the middle of the 18th century and spread to the rest of Europe and the United States. The Industrial Revolution contributed to the rise of factories, creating a demand for workers in urban areas. Over the next century, millions of people in the United States and England moved from rural areas to cities. The population of London, England, for example, grew from one million in 1800 to more than six million a decade later. Within a few decades, the so-called Second Industrial Revolution boosted urbanization in the United States. By 1950, New York City had become the largest city in the world, with a population of 12.5 million.

The Industrial Revolution also had a profound impact on cities in Asia, Africa, South America and the rest of the world. In many cases, industrialization first spread as colonizers exploited the raw materials and labor of their colonies. As one example, Mumbai, India (then called Bombay), became an important port city and financial center during Britain’s rule. Driven by British-controlled textile mills and trade, Mumbai’s population skyrocketed to over 800,000 by 1900 and then doubled to 1.6 million by 1950. Industrialization spread again as countries decolonized and became independent nations.

In recent years, urbanization has fueled the growth of ever-larger cities, even leading to a new type of city: the megacity, which is defined as having a population of 10 million or more. New York City and Tokyo became the world’s first megacities in the 1950s. By 2023, there were 34 megacities across the globe, and the United Nations (UN) predicts this number will grow to 43 by 2030. As of 2023, Tokyo, with more than 37 million residents, is the world’s largest urban area. Other megacities include Delhi, India, which has a population of more than 30 million, and Shanghai, China, Mexico City, Mexico, and São Paulo, Brazil, all which have populations well over 20 million.

Cities around the world are facing many challenges from rapid growth, particularly cities in the Middle East and other nations of Asia and Africa, which have experienced some of the highest levels of growth in the 21st century. One of the biggest challenges facing growing cities is infrastructure, including transportation systems, government services (such as hospitals and schools) and utilities. Infrastructure is important because it can affect a host of other issues. For example, a lack of investment in waste treatment and water infrastructure can lead to public-health problems, as people become sick due to the lack of adequate sanitation. Rapidly urbanizing cities often come up with innovative plans to improve infrastructure and other areas of city life. Barranquilla, Colombia, for instance, moved to improve its infrastructure by creating more public parks. This increased residents’ access to green spaces, resulted in lower crime rates and fostered neighborhood pride among residents.

Providing health services to people in growing cities is often a major challenge. Empowering residents can sometimes be a solution. In Maputo, Mozambique, for example, the local government tasked community leaders to research their neighborhood’s sanitation facilities, which were largely made up of latrines (simple toilets usually consisting of holes in the ground). This was meant to be a first step prior to the government taking further action, so at the time, there were no additional plans to change or upgrade the facilities. However, monitoring the sanitation systems increased awareness of problems, and many people upgraded their latrines without government involvement.

Other potential challenges arise from climate change. Rising sea levels and natural disasters, such as floods and wildfires, exacerbate challenges for residents experiencing poverty in urban areas. At the same time, cities can be places of innovation to combat climate change. Lima, Peru, is a megacity in the Sechura Desert, one of the driest coastal areas in the world. Although the area gets very little rain, it experiences significant coastal fog. Scientists have developed a novel system to extract water from fog, providing a sustainable solution to the lack of rain. This process not only conserves energy, but it is also more cost-effective than other methods of transporting water.

The last major challenge of growing cities is poverty. People often flock to cities in times of need. While migrants typically bring skills with them, these skills may not fit the available jobs. Difficulty finding work can contribute to and exacerbate poverty. This issue is even worse in times of conflict or drought, when the people of an entire region are affected at the same time.

Poverty and the resulting desperation are contributing factors to crime. Therefore, it is valuable for cities to address social inequity. One way cities in Finland are addressing this is through Housing First, a program that provides unhoused people with a permanent residence. While some programs require people to enter job training or drug treatment programs to be eligible for housing, Finland’s Housing First focuses on first getting people housed and then providing them with the additional support they need. Established in the 2000s, this program has helped Finland become one of the few European nations where homelessness is decreasing.

Preparing for the future means predicting the growth of today’s cities and preparing for the challenges they may face. More than half the world’s population today lives in urban areas, and the urban population is expected to increase to two-thirds of the global population by 2050. Much of this growth is due to natural increase—that is, people already living in cities and giving birth to children—but migration also plays a key role. Leaders at all levels of government must work together to prepare for and combat the issues that increased urbanization brings.