Due to growing environmental awareness and resulting concerns, refrigerants — the working fluids for refrigeration systems, heat pumps and air conditioners — have gained considerable attention. Well-known, proven fluids were banned and replaced with new ones. In the process, refrigerant mixtures were introduced to achieve acceptable properties reasonably well-matched to those of the fluids to be replaced. The thermodynamics of mixtures is considerably more complex than that of pure fluids. This text is dedicated to introducing the thermodynamic theory and background of working fluid mixtures and their practical implementations. It discusses the opportunities and challenges and aims at providing the practicing engineer, as well as the aspiring engineering student, with all the necessary information to successfully design systems that take the best possible advantage of fluid mixtures. Emphasis is placed on practical experience from laboratory investigations.