The starting point for this paper is the identification of the four necessary and sufficient conditions which a building should exhibit to be judged to be sustainable; fitness-for-purpose, cost effectiveness, environmental friendliness, and cultural significance. The contribution which the current generation of CAD tools is making to the complex human activity of designing sustainable buildings is then discussed under the headings of widening the search for solutions, integrating the decision-making process, improving design insights, differentiating objective/subjective issues and photorealistic visualization. The paper then describes developments in the next generation of IT tools, based on virtual reality, which will allow multiple designers, working in distributed locations and on different computer platforms, to design from within the virtual world.