The performance of any technology, based on the application of nanoporous materials, can never be faster than allowed by the rate of their intrinsic mass transfer. The spectrum of relevant mechanisms includes molecular diffusion as one of the most fundamental and omnipresent phenomena in nature. Discriminating its influence in relation to other mechanisms of transport resistance is thus imperative for designing and operating adsorption and catalysis processes. The chapter starts with a first principles introduction into the fundamentals of mass transfer, which is thought to address also the newcomers in the field. It is followed by a survey of the various possibilities, conditions and challenges of experimental measurement, and the pros, cons, and risks of their application. The survey ranges from the more sophisticated methods developed and exploited for an in-depth study of mass transfer in nanoporous materials such as MOFs and zeolites, up to those techniques which are also suited for the every-day use in industry, notably as a constituent of permanent quality control. The chapter is concluded with the presentation of a number of showcases illustrating the impressive progress with diffusion studies in MOFs and the remarkable gain in information which has been attained over the last few years.