Since 1950, the number of people living in urban areas has jumped from 750 million to more than 2 500 million people. Currently, some 61 million people are added to cities each year through rural to urban migration, natural increase within cities, and the transformation of villages into urban areas. By 2025, the total urban population is projected to double to more than 5 000 million people, and 90 per cent of this increase is expected to occur in developing countries (United Nations Population Division 1997).