The objectives of the Modelling Sub-Group are to unify European research in the field of computer models, to provide the best simplified models in the Passive Solar Working Group's extensive European Passive Solar Handbook and to ensure that these simple models are easy to use for design purposes. It has defined the needs of the research program for a large model and has chosen the Scottish program ESP as a large simulation model to be used as a reference for the analysis of the simplified ones. Each participant has commissioned the chosen model on his own computer and run a very simple exercise, just to verify that the program works similarily on the different machines. The analysis of the large model is being brought on its different sections, such as : input, output, heat transfer, passive solar systems, etc. The group will run ESP using the same passive solar bulding data as inputted to simplified models to assess the sensitivity of the latter ones. It will report on the work needed for the next 4 year CEC research program.