Cities play an important role in all our lives today and in the years ahead. 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". Today, the most rapid urbanization is taking place in countries in Asia, Latin America and Africa. Cities have always been at the center of economic growth, technological advances and cultural production. 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. By the 21st century, it is estimated that half the world's population will be living in cities.

The city is a part of our lives and our language; we might say, "I live in a city" or "I live near a city" or "I am moving to a city". We know what we mean when we make these statements because we have certain ideas about the city: the bright lights, the tall buildings, the traffic jams.

But, if asked, could we really define a city? Where would we draw its boundaries? How do we distinguish a city from a "town" or a "village"? There used to be a time when it was easy to identify a town or city. A town was a living space with a place of worship, like a Church, Mosque or Synagogue, a town square, a central market and a town hall. Many large cities were encircled by walls. But what about today when the old walls are no more than tourist attractions and every little settlement has a market and a town hall?

We can check the dictionary. Going to the dictionary, however, does not help us be more precise in defining the physical aspects of a city. While it tells us about the general concept of a city, it does not tell us at what point in its growth a "town" can be considered a "city" or where a "city" begins and where it ends.

The problem is that there is no agreement on these matters. Different countries, municipalities and scientists use different definitions. For example, the city of London, has less than 7 million inhabitants. But the city boundaries used to define London do not include an extended "metropolitan region". If considered as a metropolitan region, London would have a population of 12.5 million. Shanghai China, on the other hand, uses a vast area of 6,000 square km to define itself. Thirteen million people live within those boundaries, but the area covers large patches of farmland as well as some villages. Other large cities have similar problems with definitions.

In trying to define themselves, some cities use physical attributes: only an area that is "continuously built up". Others extend their definitions beyond this to include nearby settlements because the population and economy of those outlying areas are closely tied to the central city-- in this case, the definition is "economic". In yet another case, a larger area may be defined, which, as with Shanghai, would include some farmland and typically rural areas. Such a vast definition is often useful for planning and administrative purposes.