This chapter considers some thermodynamic properties of solutions in which a polymer is the solute and some low molecular weight species is the solvent. It explores the application of solution thermodynamics to problems of phase equilibrium. The chapter briefly extends these concepts to mixtures of two polymers, or polymer blends. An important fact to remember about the field of thermodynamics is that it is blind to details concerning the structure of matter. Thermodynamics is concerned with observable quantities and the relationships among them, although there is a danger of losing sight of this fact in the somewhat abstract mathematical formalism of the subject. There is probably no area of science that is as rich in mathematical relationships as thermodynamics. This makes thermodynamics very powerful, but such an abundance of riches can also be intimidating.