This review paper deals with mass transport in macromolecular materials. The study of mass transport through polymeric composite materials for example has applications in a variety of areas such as coatings, packaging and gas separations, to name just a few. Here we discuss several models for diffusion as well as several experimental techniques. in particular, we discuss models for case II diffusion, from a continuum mechanics point of view as well as via a mesoscopic theory. Variables such as temperature, molecular structure and mechanical deformation, affecting mass transport are also discussed.