Megacities worldwide are an outstanding feature of urbanization and are culprits and victims of dramatic global change processes at the same time. They do not only involve unprecedented population growth and density, but also produce a complex variety of simultaneous and interacting processes and functions, for example the tremendous consumption of resources, the production of waste and pollution or social integration/disintegration. They turn the urban habitat into both a space of risk and a space of opportunity with consequences on the prospects for achieving urban sustainability.