This paper explores the potential of geographic information systems (GIS) in the study of urban historic cartography. The first part of the paper focuses on three European schools of urban morphology, that have been using cartographic analysis in the study of the city, since the first half of the 20th century. Built on this theoretical context, the second part describes the use of GIS in redrawing historic maps of Lisbon and Oporto. A discussion of the fundamental contribution of this method is then presented. GIS encourages and enables rigorous representation of the spatial characteristics of urban phenomena. Furthermore, the application of this method provides the following five main outputs: a dynamic framework to represent the evolution of the urban form, continuously open to the addition of, and articulation with, other morphological and planning data and information; the overall and simultaneous vision of the urban-form evolution of a particular city in a long time period; the rigorous identification and characterization of urban-expansion areas; the opportunity to systematically analyse unexplored urban-development processes; and, finally, the possibility to typify the urban fabric, taking advantage of a rigorous and versatile cartographic tool.