The 1996 amendments to Section 1453 of the Safe Drinking Water Act require the states to establish and implement a Source Water Assessment Program (SWAP). Source water is the water taken from rivers, reservoirs, or wells for use as public drinking water. Source water assessment is intended to provide a strong basis for developing, implementing, and improving a state's source water protection plan. This program requires individual states to delineate protection areas for drinking water intakes, identify and inventory significant contaminants in the protection areas, and determine the susceptibility of public water supply systems to the contaminants released within the protection areas. SWAP can be used to focus environmental public health programs developed by federal, state, and local governments, as well as efforts of public water utilities and citizens, into a dydrologically defined geographic area. The Environmental Protection Agency is assisting the states in conducting source waster assessment by identifying potential sources of data and pointing to methods for assessing source waters. This report provides guidance to states, municipalities, and public water utilities for assessing source waters using geographic information system (GIS) technology. The GIS platforms can be used to organize, analyze, and manipulate available data and generate new data for source water protection areas, as well as provide capabilities for presenting the data to the public in various forms, including maps and tables. Included as appendices to this document are three case studies demonstrating the use of selected GIS-based software and hydrologic models to conduct hypothetical source water evaluations.