Geographical information systems (GISs) are a powerful means of adding value to and exploring historic data. Historic data that have been input into, and manipulated using, GIS software include (1) records that can be linked to spatial locations and (2) historic maps that have been scanned and converted to digital images. Data sources that fall into the first category may refer to point locations (e.g. a town on a large-scale map) or zones (often administrative regions) for areas at a variety of spatial scales. In terms of (2), maps may be enhanced through digitisation of the objects that the maps contain, creating separate digital layers representing features such as coastlines, streets and locations of specific places. Historic spatial data are subject to a number of particular issues that may affect how they can be used. Accuracy of the information contained in the data, in terms of both spatial (e.g. precision of survey) and non-spatial attributes (e.g. counts of people in a given category in a particular area), must be considered as must the mode of representation of the data (points, areas, etc.). Comparison of data derived from different sources that may be separated by large time periods, and that may have been generated with very different purposes in mind, is potentially highly problematic. This article reviews the key categories of geographical historical data available as well as the kinds of data-related problems faced by those who wish to explore historic environments using GIS.