The present paper reviews three-dimensional random network models of catalyst support structures. Multicomponent diffusion in the individual pores of the network is described by the dusty-gas approach. In contrast to previous publications, the present network model can be applied to any common reaction kinetics. This becomes inevitable in order to make three-dimensional network models applicable to practical problems in the industry. The present model is closely connected to measurable quantities which may be obtained by standard equipment. Examples of pore structure optimizations with respect to various optimization criteria are given. Investigations of depositions within the pores by percolation theoretical methods are described. Results of simulations are compared to measurements in a single-pellet diffusion reactor. The importance of surface diffusion is stressed. (C)1999 Elsevier Science B.V. All rights reserved.