A major project is building a specifically historical GIS (Geographical Information System) for Britain, mapping the evolving human geography of the past two centuries. This system combines the changing boundaries of administrative units with a large database of historical statistics. This paper focuses on England and Wales, where work is most advanced, and examines the sources for the mapping work, how the GIS is built, the uses of the GIS to map past geographies, and the potential for using mid-nineteenth century boundaries for mapping much earlier sources including the Domesday Book.