A summary is given of part of the CEC co-sponsored project MERGE (Modelling and Experimental Research into Gas Explosions). The objective of this part of the project was to provide improved Computational Fluid Dynamic explosion models with the potential for use in hazard assessments. Five organisations with substantial experience in both theoretical and experimental explosion modelling contributed to this model assessment study; British Gas, Christian Michelsen Institute, Imperial College, Telemark Technological Research and Development Centre and TNO Prins Maurits Laboratory. The theoretical and numerical basis of the models are described. Results are given of a comparison exercise of model predictions against calculations which were chosen to test the accuracy of the various physical sub-models embodied within the overall explosion model. The development phase of the study is also described in which further extensions to the models were made to provide the best achievable agreement with small- and medium-scale experiments also conducted as part of the project. The models were finally used to simulate large-scale explosion experiments prior to the experiments being conducted. The overall capabilities of the models are reviewed and areas of uncertainty in the physics highlighted.