An electronic (interactive) version of a 1994 publication by the US Forest Service (USFS), this site is based on the National Hierarchical Framework of Ecological Units, designed to provide "a standardized method for classifying, mapping, and describing ecological units" in the US. Ecological Subregions of the United States, contains the biophysical descriptions for the color-coded maps. This is a solid resource for anyone seeking broad ecosystem classifications for the United States.