The modern city and its buildings have been shaped over 150 years by the availability of fossil fuel, and credit. As both energy and capital become less plentiful, must cities stop evolving? On the contrary. The new engine of development in modern cities is nature herself. Rather than use massive energy inputs to create structures, heat and cool buildings, move and treat water, or grow and supply food, the bio-city works with natural processes, and with the social energy of human collaboration, to achieve the same ends. Using language such as “Paris as a sponge” or “edible London”, the bio-city is not a future dream. it is already emerging in dozens of projects around the world. Examples include: the recovery of rivers from Seoul, to Mexico City; the restoration of watersheds from Pittsburgh, to London; urban agriculture from Caracas, to Berlin; Transition Towns; “social harvest festival” events.