Sometime in 2008, the United Nations predicts, we'll reach the point at which over half of the world's people live in urban areas. More than at any time in history, the fate of humanity, our economy, and the planet will be influenced by cities.

In the period from the Industrial Revolution through the 20th century, populations in Europe, North America, and then Japan passed the half-urban point. Most of the world's future population growth is expected to come in the rapidly growing cities of Africa and Asia, two regions that are set to become predominantly urban, and in Latin America, already 77 percent urban.
As urban numbers have grown, the ranks of the urban poor have surged as well. Of the 3 billion urban dwellers today, roughly 1 billion live in "slums,"defined by UN-HABITAT as areas where people cannot secure one or more of these necessities: clean water, sanitation, sufficient living space, durable housing, or...