The urban world at the dawn of the 21st century has assumed special importance for two principal reasons. First, the world's population has recently crossed the threshold from being more rural to being more urban. In absolute terms and according to UN estimates, this means that of the world's population of approximately 6.8 billion, approximately 3.5 billion people (or 51%) are living in urban settings. To put this notion in historical context, consider that 58 yrs. (years) earlier only 29% of the world population lived in cities. The projection for 2050 is 70%. Second, a dramatic change in the urban hierarchy is taking place as the large cities—variously termed global cities, world cities, megacities, and so on—are beginning to dominate the urban landscape. This entry ...