The global population is quickly nearing eight billion people, and more and more of these people are living in cities. Today, approximately four billion people live in urban areas. This number is steadily growing, as more people move from the countryside to work in cities. This process, known as urbanization, is especially fast in African and Asian nations. 

Urbanization has taken place at different rates in various parts of the world. It became a global phenomenon only in the latter half of the 20th century. 

Metropolises are cities that accomodate over a million people. Together, metropolises can expand and grow into large megalopolises. An example of a megalopolis is the BosWash city chain in the northeastern United States, which is home to tens of millions of people. 

In Europe, the largest population centers are located near the metropolises of London, Paris and Moscow, as well as in the German region of Ruhr.

The largest Finnish population center is the Greater Helsinki region, which is home to approximately 1,5 million people.