Urbanization, or the process by which cities grow, is one of the most important geographic phenomena in the world today. This is because the proportion of the world's population living in urban settlements is growing at a rapid rate, and many of the most significant economic, social, cultural, political, and environmental processes are increasingly occurring within and between the growing numbers of cities in the world today.

According to the United Nations (UN), by 2003, there were already 372 cities of a million or more people, 39 cities with over five million residents, and 16 cities over 10 million. Projections for the future suggest that, in 30 years, about 60 percent of the world's population will live in cities, though what defines a “city” and “urban” ...