Cities are places of human convergence, where people live, work, and play. But beneath the bustle of any city are systems that make these hubs of humanity function. Cities are akin to living things that take in energy, metabolize material, and spit out waste. They consume and grow, using digestive, respiratory, and circulatory systems. And, like living things, cities can, with a nudge from citizens and their leaders, evolve in directions that increase their prospects for survival.

Informed citizens and city leaders understand these systems and how they can be  designed to advance human well-being. In a world of ever-rising consumption of energy, materials, food, and water, modern urban systems are ripe for critical review, and for exploration of more-sustainable solutions.