The pursuit of urban sustainability poses problems of how to evaluate proposals for new development. Without an agreed basis for comparing alternatives, it may be difficult to justify one set of proposals over another. Mathematical modeling, by quantifying aspects of the urban environment, is often promoted as the most credible way of assessing sustainability. But the use of models in practice raises many issues that need to be considered if models are to gain widespread acceptance. Four United Kingdom universities are researching an energy and environmental prediction (EEP) model to predict energy consumed and emissions produced by the built environment and traffic. As well as further technical development of the model, the research aims to study how the model can be used by different organizations to promote sustainable decision making. In this short paper we examine the role that models in general can have in supporting decision making about urban sustainability.