AbstractAs the digital revolution deepens and pervades every aspect of daily life, virtual realities begin to penetrate one another in a multiplicity of ways. The amount of sensing data being compiled on the city grows, enabling the construction of virtual realities that can, in turn, be transformed for diverse purposes. Here, Michael Batty and Andrew Hudson‐Smith from the Centre for Advanced Spatial Analysis, University College London, outline how they went about the construction of a virtual city in central London. A conventional 3‐D‐GIS/CAD model was used as the basis on which to build a digital realm in which designers are cast as avatars and populations as agents, so as to define new ways in which to understand and plan the city. Copyright © 2005 John Wiley & Sons, Ltd.