Life relies on mutualistic relationships among species, and on the constant rejuvenation of Earth’s materials. Mutualistic cities would do the same thing, enhancing biodiversity, clean air, better soils, fresh water, and stronger communities. Today, however, cities are far from mutualistic. Currently, more than 4 billion people live in cities, and that number is rising quickly. These conglomerations of humanity consume vast Earth resources, and, worst yet, disgorge astonishing amounts of waste into the atmosphere, water, land and sea around them. Unlike "smart cities" that rely on sophisticated technology to monitor and respond to environmental conditions, and unlike "sustainable cities" that stress reduction and reuse, the concept of a "mutualistic city" emphasizes regenerative cycles and virtuous feedback loops. These cities are the key to our future.