This chapter reviews fundamental concepts from thermodynamics that are essential to the aerodynamic design and analysis of turbines. Thermodynamics provides the basis for defining and evaluating the energy transfer processes. It also contributes the equation of state for the working fluid, which is one of the governing equations of fluid flow through turbomachinery. The requirements for the equation of state are particularly demanding for turbines. The commonly used ideal gas equation of state does have applications, such as some turbocharger and gas turbine applications. But many industrial turbines are applied to nonideal working fluids such as steam and various hydrocarbons. This writer's general ideal∕nonideal equation of state model is suitable for this purpose [1, 2, 6, 7]. The previous descriptions of this model addressed compressor aerodynamics applications. Unlike compressors, two-phase flow is not at all uncommon in turbines. Some extensions to that model are needed to treat cases where liquid is present.