In April 1998, Environmental Protection Agency introduced the new generation of its Web-based mapping tool, EnviroMapper. Since then, EnviroMapper (http://maps.epa.gov) has become one of the most visited sites for the agency. The application has an average of 40,000 hits per day and more than two million times each month. EnviroMapper gives users the power to query one million EPA-regulated facilities through a Web browser; it also generates text reports of environmental information. EnviroMapper can be used to view and query environmental statistics, profiles, and trends; environmental information for U.S. metropolitan areas; environmental information for watersheds; and Superfund National Priorities List (NPL) sites. The geospatial data that powers EnviroMapper is stored in EPA's National Shapefile Repository, which contains more than 20 gigabytes of spatial data. This repository utilizes technology called Virtual Grid, which allows the data to be retrieved efficiently through the Web. The latest version of EnviroMapper takes advantage of Esri's Spatial Database Engine (SDE). EnviroMapper can access multiple SDE databases that are at distinct remote locations. OpenLink, another EnviroMapper feature, allows users to request map images directly from EnviroMapper, via HTTP protocol, and integrate them within their own Web pages. OpenLink Wizard facilitates OpenLink by automatically generating the HTTP protocol based on user-entered criteria, which means that less-technical users can enhance their Web pages with EPA's environmental information. This "open architecture," which affects both data and application architecture, allows EPA to come closer to achieving its goal of making EPA information readily available to the public. This work was supported by the EPA Office of Information Resources Management Mission Oriented Systems Support contract under the Envirofacts Information Warehouse Initiative Task Order (TO #13).