The percentage of impervious surface area in a watershed has been widely recognized as a key indicator of terrestrial and aquatic ecosystem condition. Although the use of the impervious indicator is widespread, there is currently no consistent or mutually accepted method of computing impervious area and the approach of various commonly used techniques varies widely. Further, we do not have reliable information on the components of impervious surfaces, which would be critical in any future planning attempts to remediate problems associated with impervious surface coverage. In cooperation with the USGS Geographic Analysis and Monitoring Program (GAM) and The National Map, and the EPA Landscape Ecology Program, this collaborative research project utilized very high resolution imagery and GIS techniques to map and quantify the individual components of total impervious area in six urban/suburban watersheds in different parts of the United States. These data were served as ground reference, or "truth," for the evaluation for four techniques used to compute impervious area. The results show some important aspects about the component make-up of impervious cover and the variability of methods commonly used to compile this critical emerging indicator of ecosystem condition. ?? 2004 by V. H. Winston and Sons, Inc. All rights reserved.