This paper presents a detailed evaluation of two- and three-dimensional numerical models of flow and heat transfer over louvred fin arrays in compact heat exchangers; Two 3-D models are described, both of which incorporate the effects of tube surface area and fin resistance on the overall heat transfer rate. Both of these features lead to a lowering of the predicted heat transfer rate per unit area compared with the 2-D model and, as a result, the 3-D models give predictions of overall heat transfer in better agreement with experimental observations. All of the models give accurate predictions of pressure losses, but it is argued that the superior heat transfer predictions of the 3-D models make them much more useful as design tools than 2-D models, even : though they require much greater computing resources