Mass transfer in micro- and mesoporous materials is of crucial importance in their practical application for separation and catalysis, since the mobility of adsorbed molecules ultimately limits the rate of the overall processes. Diffusion, i.e. the irregular thermal motion of the molecules, is the dominating process. Diffusion measurements are therefore indispensable for the evaluation of the quality parameters of porous materials. Due to their ability to directly follow the diffusion path of the molecules, microscopic techniques are to the forefront, amongst the various methods of diffusion measurement. Besides describing their fundamentals, this contribution describes the application of these techniques to investigate structure-mobility-relations in zeolites and in mesoporous materials of type MCM-41. Some features of particular technological relevance, e.g., the phenomena of correlated diffusion anisotropy, of single-file diffusion and of molecular traffic control, are discussed in detail.