A radical demographic shift has occurred worldwide, characterized by the movement of people from urban to rural areas at an increasing rate. While in the mid-1970s less than 40 percent of the world’s population lived in urban areas, it has been estimated that by 2025, 60 percent of this population will do the same (World Bank 2000). The world’s urban population today is around three billion people, about the same size as the world’s total population in 1960 (Satterthwaite 2006). Around 50 percent of the world’s population now lives in urban centers, compared to less than 15 percent at the beginning of the century.