Recent developments involving diffusion studies are discussed. A method for predicting the diffusivity of ethylene in polyethylene has been developed. Results compared to experimental values indicate the method is quite reliable. A correlation that combines the analytical solutions for diffusion in a plane sheet and an infinite cylinder is presented. It offers a useful analytical tool for system commonly encountered when dealing with extruded polymers, which often have the form of short cylinders. Experimentally, the static sorption technique has been modified for extension to diffusion studies. Through running multiple capsules in parallel, diffusion coefficients as well as solubilities can be determined. Issues involving heat transfer and thermodynamics in the static sorption technique are discussed.