Cities, large and small, are at the heart of a fast changing global economy—they are a cause of and response to world economic growth.

The world's cities are growing because people are moving from rural areas in search of jobs, opportunities to improve their lives and create a better future for their children.

This is the first time in human history that the majority of the world's population lives in urban areas.

• 3 billion people—half the world's population—live in cities.
  
• Two-thirds of all people will live in cities by 2050. (In 1800, only 2% of people lived in cities and towns. In 1950, only 30% of the world population was urban.)
  
• Almost 180,000 people move into cities each day.
  
• 60 million people move into cities each year in developing countries. This rate of movement will continue for the next 30 years
  
• Over the next 15 to 20 years, many cities in Africa and Asia will double in size.

City populations are growing faster than city infrastructure can adapt.

Many urban areas are growing because their rural hinterlands are depressed, which forces impoverished rural people to move to the cities in search of work.

These newcomers often end up not finding the opportunities they are looking for, but become part of the urban poor. Upon arrival to the city, they often encounter:

• Lack of housing: To make up for the lack of available homes, newcomers often set up shelters on city outskirts, usually on public owned land. This land tends to be dangerous and inhabitable, such as flood plains, river banks, steep slopes or reclaimed land.
  
• Lack of infrastructure services: Slum dwellers often live without electricity, running water, a sewerage system, roads and other urban services.
  
• Lack of property rights: As illegal or unrecognized residents, slum dwellers have no property rights to the land they inhabit, which make it impossible for them to use land as collateral.

Over the last 50 years the global population living in slums has risen from 35 million to over 900 million. This number could double in the next 30 years.

Slum dwellers make up the majority of the urban population in Africa and South Asia. More than half the world's poor will live in cities by 2035, according to some estimates.

In 1950, New York was the only city in the world with a population of more than 10 million inhabitants.

People living in slums are at particular risk to disease: On top of dealing with pollution from dirty cooking fuels, primitive stoves, and poor access to water and sanitation, they are exposed to modern environmental hazards, such as urban air pollution, exhaust fumes and industrial pollution.

Also, as cities grow so do environmental problems:

Air quality worsens in cities. Each year 1 million people die from urban air pollution.

Traffic increases, leading to more congestion and more road accidents. Half a million people die and 15 million are injured in urban traffic accidents in developing countries each year, according to the World Health Organization. Victims are mostly poor pedestrians and bicyclists. Those who survive are often left disabled. For example, in Bangladesh, it is reported that nearly 50% of hospital beds are occupied by road-accident victims