At the beginning of the new millennium humanity is involved in an unprecedented experiment: we are turning ourselves into an urban species. Large cities, not villages and towns, are becoming our main habitat. The cities of the 21st century is where human destiny will be played out, and where the future of the biosphere will be determined. There will be no sustainable world without sustainable cities. How can we make a world of cities viable in the long term – environmentally, socially as well as economically?