As of 1975, there were three metro areas with at least 10 million people — Tokyo, New York and Mexico City. A list of the 10 largest cities at that time would have included Paris (now 25th), Moscow (22nd) and Los Angeles (21st). Now there are 31 megacities with at least 10 million people, and most of them are in the developing world. The UN projects 10 more will join the list by 2030, and all but one (Bogotá) is in Africa or Asia.
The Big Picture: 518 million people (7% of the global population) now live in megacities of 10 million or more people. That's a tenfold increase from four decades ago, and it's radically changing the way people live, work and view the world.