At the beginning of this new millennium, the earth has arrived at a historical point of inflexion. For the first time in history the world is becoming predominantly urban with more people living in cities than in rural areas. Many municipalities are hardly able to keep pace with the speed of the urban growth and face great difficulties to provide sustainable infrastructure to their citizens. This is particularly true for mega-cities in developing countries that are most affected by these rampant urbanisation trends. The character of urban growth is often informal (unplanned) and takes place predominantly in peri-urban areas and the city fringes, resulting in a high number of unserved people, who are exposed to severe health and environmental risks.