The population of the world recently reached 7 billion.

Over half of these 7 billion people live in cities.

Previously, most people lived on farms or in small towns.

But now cities are growing larger as people leave farms and towns.

Today, nearly 30 cities in the world have populations of over 10 million people.

Tokyo, the world's largest city, has a population of about 37 million people.

There are several reasons why cities are growing.

One reason is that big cities are major business centers.

Many factories, offices, and stores are located in big cities.

These businesses need workers.

People are moving to cities so they can find good jobs.

They want jobs that pay enough money to support themselves and their families.

Many big cities are also major ocean ports.

Port cities have grown because international trade has been increasing.

Ships bring materials into the city.

Workers are needed to unload the materials and deliver them to factories and stores.

Big cities are also major transportation centers.

They have airports, train stations, and bus terminals.

Convenient transportation enables visitors to come to the cities to live or for shopping, business, or vacations.

Finally, big cities are likely to have large universities and technical schools that attract students to study there.

As cities grow, they face several problems when large numbers of people live in a limited area.

For example, new buildings must be constructed for families to live in.

Growing businesses need new buildings, too.

However, there is usually very little empty space within the area of a large city.

As a result, even a small plot of land is expensive.

For this reason, builders are removing small, one-story buildings that were constructed in previous times.

They are replacing them with tall, vertical buildings.

As an example, a one-story apartment building might provide living space for 25 people.

A ten-story building occupying the same land area could provide living space for 250 people.

Many big cities have made efficient use of their land by going vertical.

Hong Kong, for example, has nearly 8,000 structures that are 12 or more stories in height.

New York City and São Paulo have nearly 6,000.

Singapore and Moscow each have about 4,000.

Another problem cities face is designing and installing complex systems to provide services such as water and electricity to new homes and businesses.

An efficient system must also be designed for collecting tons of trash and disposing of it in a safe way.

Finally, a growing population means more cars, trucks, and buses in the streets.

Narrow streets in old cities are often unable to handle today's increased traffic.

In addition, smoke from vehicles causes serious air pollution in some cities.

To reduce traffic and pollution, some cities have passed laws that restrict the use of automobiles in certain areas.