This chapter discusses solubility of polymers and supercritical fluids (SCF). Mixtures of polymers and supercritical fluids (SCFs) have many features in common with mixtures of polymers and conventional incompressible liquids. As a result, a variety of operations, such as polymerization, dissolution and precipitation, swelling, fractionation, plasticization, impregnation and extraction, can be carried out on polymers using SCFs. This chapter primarily compiles experimental data on the pressure dependence of physical properties of fluid phase polymer-SCF mixtures. Phase equilibria are addressed, including the solubility of polymers in SCFs, the solubility of SCFs in liquid polymers, and the three-phase solid-fluid-fluid equilibria of crystalline polymers saturated with SCFs. Melting point depressions of polymers in the presence of SCFS are explained. Additional thermodynamic properties include glass transition temperature depressions of polymers, and interfacial tension (IFT) between SCF-swollen polymers and the SCF. The viscosity of fluid phase polymer-SCF mixtures is also explained. Values of IFT are measured by analyzing the shape of a pendant drop. Viscosity reductions measured due to the presence of an SCF in the polymer are listed.