This chapter presents the chemical, physical, and technological properties of the synthetic fluids as well as the advantages and disadvantages of some important synthetic fluids. Regarding the limits of application of fluids, physical and chemical effects have to be distinguished. The physical effects are characterized by temperature and pressure. Synthetic fluids may be classified based on their chemical composition. The temperature limits of some synthetic fluids. The temperature limiting the oxidation and the thermal stability was plotted against the service life of the fluid. For application reasons, a comparison of certain property groups seems to be most useful to select the optimum synthetic fluid to cover the requirements of special tribological contacts. An application of synthetic lubricants strongly depends on their specific properties, especially concerning those of mineral oils. The simultaneous comparison of thermal, oxidation, hydrolytic, and volatility behavior of synthetic fluids and of mineral oil.