The selection a working fluid as a refrigerant for the vapor compression cycle is discussed. For thermodynamic performance to be optimized particular attention must be paid to the fluid’s critical point and molar heat capacity. To achieve a reasonable balance between volumetric capacity, which implies equipment size, and coefficient of performance, which is efficiency, it is necessary to select a fluid with a critical point such that the application’s condenser will operate in a reduced temperature range of approximately 0.7 to 0.9. It is also necessary to select fluids within a rather limited range of molar heat capacity values. The specifics as to how these properties effect the cycle’s performance are presented in detail. For a better matching of the working fluid to a given application, both azeotopic and zeotropic mixtures of fluids are used. The characteristics and performance advantages and disadvantages of these mixtures are discussed.