The particularities of the glassy state of polymers require an in-depth study of the system in order to understand the differences from the better characterized rubbery state. The applications for glassy polymers are very numerous and wide-ranging. Thermodynamic theories try to explain the properties and anomalies found. The different models proposed in the literature for solubility of solutes in glassy polymers are reviewed with regard to their ability to reproduce the published experimental data, mainly for the carbon dioxide/polycarbonate system. The CSPSS has studied the carbon dioxide/polylactic acid (PLA) system. PLA is a polymer that is frequently used in drug delivery applications and that has a low glass transition temperature.