In engineering practice, we routinely encounter the diffusion of small molecules through solid polymers, the diffusion of polymer molecules in dilute or concentrated solution, and the transport of macromolecules through polymer melts. We came across diffusion in a dilute solution when we developed the theory of the ultracentrifuge in Chapter 8 as a method of determining polymer molecular weight. Similarly, solution polymerization involves diffusion in a concentrated solution. The reverse situation of the mass transfer of small molecules through polymers has great technological importance. Thus, anisotropic cellulose acetate membranes can be used for desalination of water by reverse osmosis [1], and ethyl cellulose membranes can be used to separate gas mixtures such as air to yield oxygen [2]. Other common situations include the drying of polymeric coatings [3] and the removal of the monomer and other unwanted volatiles from the finished polymer by the process of devolatilization [4]. In the field of medicine, polymeric drug delivery systems have become a reality [5]. For example, there is now a commercially available implant for glaucoma therapy, consisting of a membrane-controlled reservoir system made from an ethylene–vinyl acetate copolymer [6]. This implant is placed in the lower eyelid’s conjunctival cul-de-sac, and it delivers the drug pilocarpine continuously over a 1-week period; normally, patients would receive eye drops of this drug four times each day. A few other examples involving diffusion through polymers are biomedical devices, such as blood oxygenators and artificial kidneys. Finally, polymer diffusion in polymer melts is relevant to the self-adhesion of polymer layers. Polyimide layers, for instance, are used as insulators in electronic packaging, and the peel strength of such a bilayer is found to correlate with the interdiffusion distance [7].