To an observer in space, humanity's footprints on the surface of the Earth are large and varied. They include the regular patterns of irrigated cropland, straight lines of roads and railways running across continents, reservoirs on river systems, and the cement rectangles of ports and seawalls along coastlines. But what about humanity's signature footprint-cities? By day, cities viewed from space can blend into the countryside, or appear as gray smudges, depending on the style of development and size of the urban area. At night however, city lights present the space observer spectacular evidence of our existence, our distribution, and our ability to change our environment.