This paper describes the theoretical basis and application of a computer model developed for analysing the thermal performance of vehicle heating and cooling systems. The model has a modular structure such that component submodels can be added, modified or changed in order to conduct parametric studies to examine the effects of various design and operating parameters on thermal performance. The model can provide useful information at the early stages of the design of the cooling system by enabling the development engineer to predict performance trends. Details of each submodel are described together with the overall structure of the vehicle model. The model application is shown by presenting a few test cases and model predictions are compared with measured data. The future potential of the model, as a design analysis tool, and its role in vehicle development programmes are also examined.