Agreed intergovernmental targets to reduce the emission of greenhouse gases due to predominantly urban anthropogenic activities are becoming increasingly stringent; one might even say ambitious. One crucial strategy in achieving these targets will be to improve the efficiency with which resource are consumed with our cities, which now accommodate more than half the global population. An important tool for testing whether governments are on track to reach their targets will be computer models which simulate the energetic consequences of alternative urban planning and design scenarios. In this paper we describe progress that has been made to develop one such tool: CitySim. In particular we describe the graphical user interface with which urban scenes may be described and the solver that simulates the energy flows within these scenes. We also describe work that is both underway and planned to enable users to simulate urban resource flows in a more comprehensive way and at a variety of scales, from the neighbourhood to that of the City