Recent advances in computer-visualization technology have brought urban historians new tools for analyzing the growth of historic cities. This paper examines both the prospects and problems involved in using this technology to map the development of urban form. Using a computer model of Cairo in two different periods of the Middle Ages, the author has attempted to reconstruct the physical reality of the city and to animate the city so that modernday observers may experience its principal streets. While the work has shown computer simulation to be a significant tool for helping urban historians understand the built environments of the past, it has also exposed possible pitfalls in the seductive potential of such simulations.