We often think of the city as a "modern" or recent development, but cities have existed for thousands of years and have their roots in the great river valley civilizations of Mesopotamia, Egypt, India and China.

Although we might know or sense what a city is, there is no exact definition of its boundaries, of where it starts and where it ends.

In the past, walls may have defined a city. In many ancient cities you can still see the ruins of those walls, but the walls no longer mean anything.

How do you define a city today?

Take 5 minutes to discuss this question with your classmates.

How do you define a city today? Take 5 minutes to discuss this question with your classmates. No matter how you define a city, however, there is agreement that cities play an important role in all our lives today and in the years ahead.

Questions like this can cause inaccuracies and disagreements. For example, depending on the boundaries used, Tokyo, Japan, can have a population of anywhere between 13 and 36 million.

After the industrial revolution, urban centers grew rapidly and over the past 50 years there has been an "explosion" in the growth of cities, both in their numbers and in their size -- this is called "urbanization".

Cities have always been at the center of economic growth, technological advances and cultural production. Raphael. The School of Athens Vatican City.

But their rapid growth has also brought negative things: urban violence and poverty, homelessness, overcrowding and health problems, pollution and waste.

Cities have had a great impact on our lives and on world civilization in general. They are becoming more and more important as their sizes and numbers grow. As of April 2010, more than half of the world's population is now living in cities.

Phase 1 The first phase began between 5 to 6 thousand years ago with settlements that grew into what we call the river valley civilizations of Mesopotamia (present-day Iraq), Egypt, India and China.

Phase 1 Early on, the settlements depended largely on agriculture and domesticated animals. However, as the civilizations grew in size and trade routes grew in number, these settlements became centers for merchants, craftspeople, traders and government officials

Phase 2 The second phase in the development of cities came much later with the industrial revolution in Europe around the middle of the 18th century.

Phase 2 Factories needed a large labour force and a rise in commercial activity created new opportunities in cities. Looking for employment and a better life, people moved from rural areas into cities in greater numbers than ever before.

Phase 3 The third phase began after the Second World War. The largest and fastest growth in the world's urban population has taken place in the decades since 1950.

Phase 3 As the world economy became more international and grew in size, cities all over the world began to grow larger at a very fast pace. Most of this growth has been concentrated in Asia, Latin America and Africa.

Cities often get a bad reputation for being chaotic, crowded places. We will see that cities have serious problems to cope with, but they are also very efficient places that provide services to thousands and sometimes millions of people.

No matter where you live, you need some essential things to survive: shelter (housing), food, and water. Life in a city requires more than that.

Electricity for buildings and streets; a safe way of dealing with the garbage; transportation so that people can get from one place to another.

Citizens also need schools so that they can get an education; places for recreation such as museums, sports arenas, and concert halls; shops so they can buy what they need, from clothing to food. These are "services" available in a city.

Citizens also require parks and green spaces. Beaches, if available, and other outdoor facilities to meet recreational and active needs.

The physical building components required to provide all these services are referred to as infrastructure.

It is a tremendous task for a city to provide the necessary infrastructure and services to its citizens. Some of this task is handled by city ("municipal") governments, some of it by private groups, such as businesses or neighborhood organizations.

The advantage of a city is that it has a greater population density than rural areas, which means that many people are concentrated in a small space rather than being spread out over a large territory.

This allows the government and others to provide more services to a larger number of people. One electricity line to a single neighborhood can serve hundreds or thousands of people.

Measure the area of your classroom and calculate its population density. Explain why a high population density can be an advantage for a city. In what cases would it lead to problems? (You can use the example of your classroom to clarify your answers.)

Which of the city's services (including cultural services) do you use during the week? Next to each write the infrastructure necessary for the service, (e.g. Education : school buildings/ water : plumbing, etc.) Are there any problems with the services or infrastructures in your town or city? Do you think they could improve? How?