The world's fastest-growing population is also its most rapidly urbanizing one. According to a U.N. report, within 4 decades, Africa's cities will triple in size, with 60 percent of the continent's people living in urban rather than rural areas. This shift will present huge challenges to the formerly rural continent and its largely impoverished governments, which will need to oversee the construction of adequate housing, infrastructure, transportation, and sanitationsystems. Already, 200 million people live in slums in sub-Saharan Africa. But development experts believe urbanization can improve people's standard of living if it is accompanied by adequate planning and building. They say sub-Saharan countries can learn from the example of their neighbors in North Africa, where Egypt, Libya, Morocco, and Tunisia, have reduced the populations of their slums by half in the last 20 years.