Porous materials are used extensively in the petroleum and chemical process industries as catalysts and adsorbents. Of the various contributions to the flow of fluid through these materials, diffusion is the most important, since more often than not, it is the rate determining process. To facilitate the design of improved catalytic and adsorption processes, a greater understanding of the complexities of diffusional behaviour, particularly at the molecular level, is required. Computer simulation is ideally suited to this goal, providing a direct link between the microscopic properties of molecules and macroscopic properties, which are measured in the laboratory.