||Among the most visually compelling images of the whole Earth have been those created using data obtained at night by astronauts or from satellites. The proliferation in use of electric lighting—including from industrial, commercial, municipal, and domestic sources—is striking. It sketches the spatial distribution of much of the human population, outlining a substantial proportion of the world's coastline, highlighting a multitude of towns and cities, and drawing the major highways that connect them. The data embodied in these nighttime images have been used to estimate and map levels of energy use, urbanization, and economic activity. They have also been key in focusing attention on the environmental impacts of the artificial light at night itself. Explicit steps need to be taken to limit these impacts, which vary according to the intensity, spectrum, spatial extent, and temporal dynamics of this lighting.