Two established engineering models for predicting environmental noise levels, the European "Harmonoise" and the Scandinavian "Nord2000", have been compared for a number of test cases involving a variety of source and receiver configurations, terrain profiles, noise barriers, properties of the ground, and meteorological conditions. The predictions are compared with reference data from the literature, and with new experimental results. In most cases the models agree very well, but in some cases the predictions deviate significantly from each other. There are many similarities between the approximations used in the two models, but some parts of the modelling methods are clearly different. The most significant differences between the two modelling methods are pointed out, and it is attempted to explain the observed differences between the predictions.