The year 2007 was a turning point for the world, marking the first time when the majority of the global population lived in cities rather than in the country.
The world's population is expected to surpass 9 billion by 2050, and increasing urbanization will push the urban-rural divide even further.
Do the world's cities have the jobs, infrastructure and space to support this kind of growth? The answer might be found in the explosion of world slums over the past decade. The United Nations predicts that 2 billion people worldwide will live in slums by 2030.
In his 2006 book "Planet of Slums," urban historian Mike Davis paints a dark picture of the future to come, writing:
The cities of the future, rather than being made out of glass and steel as envisioned by earlier generations of urbanists, are instead largely constructed out of crude brick, straw, recycled plastic, cement blocks, and scrap wood. Instead of cities of light soaring toward heaven, much of the twenty-first-century urban world squts in squalor, surrounded by pollution, excrement and decay.
Worldfocus.org's weekly radio show explored urbanization and the rise of slums, examining how such deplorable conditions might be addressed, even as the global economic crisis looms.
Worldfocus anchor Martin Savidge hosted a panel of guests.
Erhard Berner is an associate professor of developmental sociology at the Institute of Social Studies in The Hague. He has done extensive research on urban poverty and community responses in the Philippines and elsewhere and served as a consultant to UN-Habitat, NGOs, and government institutions.
Robert Neuwirth spent two years living in shantytowns across the developing world to write "Shadow Cities: A Billion Squatters, A New Urban World." He is now at work on a book chronicling the global reach of the informal economy.
Mary Wiltenburg is an independent reporter, now following a year in the life of a refugee family in the U.S. and Tanzania in a series called Little Bill Clinton, a real-time multimedia project with The Christian Science Monitor and the Pulitzer Center on Crisis Reporting.
Below, view a slideshow of life in five major world slums.