Room metaphors have become increasingly popular as a basis for CSCW systems. The paper describes how such metaphors might be extended to support large scale communication through the introduction of a spatial model for mediating conversations in virtual computer spaces. The model is described in terms of an abstract mathematical framework and the paper then outlines how this might be applied to various kinds of CSCW system. As a next step, the combination of rooms into larger virtual structures is considered and this results in proposals for structuring, navigating and mapping a large virtual Cyberspace for cooperation. Finally, the paper describes a current prototype application and reflects on some of the architectural issues involved in its realisation as a distributed system.