The Digital Atlas of Roman and Medieval Civilization (DARMC) makes freely available on the internet the best available materials for a Geographic Information Systems (GIS) approach to mapping and spatial analysis of the Roman and medieval worlds. DARMC allows innovative spatial and temporal analyses of all aspects of the civilizations of western Eurasia in the first 1500 years of our era, as well as the generation of original maps illustrating differing aspects of ancient and medieval civilization. DARMC's coverage begins under the Roman empire and extends nearly a thousand years toward the present by encompassing the medieval world. Although the initial post-Roman focus has been on medieval Europe, Byzantium and the Crusades have not been neglected, and we have begun to include the essential third leg of the tripod of medieval civilization, the Islamic world. DARMC contains dozens of data layers in several geodatabases, that is, databases whose data is georeferenced or specified in terms of decimal degrees of latitude and longitude.