Adequate housing for the urban poor is looming as a major challenge for the 21st century. According to UN-Habitat (2006), there were almost a billion slumdwellers in the world in 2005. If current trends continue, the slum population will reach 1.4 billion by 2020. Recent analyses of satellite images corroborate the leading role of informal occupations in the rapid geographical extension of cities in the developing world (Angel et al, 2005).