The paper describes the basic principles and considerations used in modeling for evaluation and scale-up of steam reforming technology. The complicated interaction between catalyst, heat transfer and pressure drop is necessary to know and fully understand when reactors are designed. Although formulated more than 50 years ago, the two-dimensional heterogeneous reactor model forms the basis. But today's highly integrated reactor designs and optimization of catalyst shapes require a detailed understanding of the complicated flow patterns in steam reforming reactors and this requires use of computational fluid dynamics. This challenges the correctness of the basic key correlations for kinetics, heat and mass transfer and pressure drop. Model evaluation requires fitting of parameters to reconciled process data, but discrimination among different rigorous reactor models is difficult due to the limited number of measurements in the reactor. (C) 2016 Elsevier B.V. All rights reserved.