This lecture introduces the design and selection of working fluids for heating, cooling, and power generation, focusing on the thermophysical properties of working fluids. It covers the evolution of refrigerant technology, the EU F-Gas Regulation, opportunities presented by regulations, and the future of refrigeration technology. The lecture emphasizes the importance of balancing performance, safety, sustainability, and total cost of system ownership when selecting refrigerants. It also discusses the impact of regulations on innovation, safety, and the circular economy.