The National Historical Geographic Information System (NHGIS) is a five-year NSF-funded project designed to create a comprehensive U.S. census database-at the census tract and county level-for both the geographical and attribute data. Technological change presents an unprecedented opportunity to make these data readily available for social science research; thus bringing the complete census within reach of social scientists unlocking the potential of two centuries of data collection, and stimulating research in economics, history, sociology, geography and other fields.