The use of supercritical carbon dioxide as a processing solvent for the physical processing of polymeric materials is reviewed. Fundamental properties of CO2/polymer systems are discussed with an emphasis on available data and measurement techniques, the development of theory or models for a particular property, and an evaluation of the current state of understanding for that property. Applications such as impregnation, particle formation, foaming, blending, and injection molding are described in detail including practical operating information for selected topics. The review concludes with some forward-looking discussion on the future Of CO2 in polymer processing.