The absence of reliable and theoretically consistent correlations for the prediction of the thermohydraulic performance of Plate-fin surfaces from the definition of surface structure is placing a constraint upon plate-fin exchanger design. Many workers have made direct use of the experimental data without fully appreciating the limitations imposed by the way in which the experiments have been conducted. The dangers of such a practice are exposed. The data of Kays and London are then re-analyzed using established heat transfer theory The comparison between the predicted and reported friction factor and j factor data is good. The predictions compare favorably with experimental measurements over laminar transitional, and turbulent flow regimes. Predictions for heat tran, fer in the transitional flow regime can be improved if the transition occurs at a lower Reynolds number The new equations can be used in design with greater confidence than the use of experimental data alone.