The Forum on Geologic mapping applications in the Washington-Baltimore urban area was convened on April 23, 1997, at Reston, Virginia. The forum was cosponsored by the U.S. Geological Survey and the Maryland Geological Survey, with assistance from the Virginia Division of Mineral Resources. Spatial earth science information in the Washington-Baltimore area provides a scientific framework for environmental assessment, urban planning, and future resource and hazard investigations in this area of the Chesapeake Bay watershed, which has sustained three centuries of urban growth.