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Half of the world’s inhabitants (3.3 billion people) now live in towns and cities. And most of the world’s urban dwellers live in developing countries, where populations are largest. Indeed, one in two live in Asia, home to two-thirds of humankind. Over the last fifty years, urban populations have been growing much faster in poorer countries than in the richest ones (4.3% per year on average in Africa, 1.2% in Europe), and 15 of the world’s 20 largest cities are in developing countries. While urbanization is historically a driver of economic and social progress, the rapid urban expansion in Southern countries, and the housing, employment and transport problems associated with this growth appear to be holding back their development.