Polymer materials are used today in a wide range of technological applications. The accurate knowledge of physical properties of the polymer systems determine to a large extent, whether a given polymer is suitable for a specific application. At the other-hand side, the accurate knowledge of the thermodynamic properties of the systems is of importance for the optimum design of industrial polymer processes. Examples are: - polymerizations in supercritical media - separation of the polymer from reaction mixture - devolatilization of low-molecular-weight substances (solvents or monomers) from polymers - fractionation of polymers with respect to molecular weight as well as to comonomer composition (for copolymers) - separation of two polymers from each other - impregnation/dyeing of polymers - separation of high-volatile substances using polymer membranes However, experimental data of polymer solubility is often scarce. Considerable experimental effort is generally required for determining the phase equilibria of polymer systems. Here, thermodynamics can provide a powerful and robust tool for modelling and extrapolation of experimental data. The talk gives an overview about experimental determination as well as modeling of phase behavior in polymer systems. Polymer solubility is discussed as function of temperature, pressure, polymer concentration, and molecular weight. In the case of copolymers, also the influence of copolymer composition is considered.