This chapter summarizes the thermodynamics of multicomponent polymer systems, with special emphasis on polymer blends and mixtures. After a brief introduction of the relevant thermodynamic principles -laws of thermodynamics, definitions, and interrelations of thermodynamic variables and potentials -selected theories of liquid and polymer mixtures are provided: Specifically, both lattice theories (such as the Flory-Huggins model, Equation of State theories, and the gas-lattice models) and off-lattice theories (such as the strong interaction model, heat of mixing approaches, and solubility parameter models) are discussed and compared. Model parameters are also tabulated for the each theory for common or representative polymer blends. In the second half of this chapter, the thermodynamics of phase separation are discussed, and experimental methods -for determining phase diagrams or for quantifying the theoretical model parameters -are mentioned.