Prior to 2008, most of the world’s population lived in rural areas. Since then, and perhaps for the rest of human history, cities will be the dominant form of human habitation on this planet. It is therefore imperative that urban places are healthful and desirable. But how does one know if a city is healthy? What are the “vital signs” that indicate that everything is okay? These manufactured environments require enormous energy and material resources and create tremendous quantities and concentrations of wastes. Are they sustainable? What are the dials, gauges, and warning lights that should be on every big city mayor’s dashboard to properly monitor these metropolitan machines? Can these indicators be used to make better decisions about how to create and sustain cities?