Hydraulic fluids that are used in the majority of industrial applications have traditionally been based on mineral oil, but the properties of oil do not meet every technical requirement. This chapter examines the different types of “fire-resistant” hydraulic fluid currently in commercial use and their properties and limitations in service. In the United States, the Bureau of Mines also introduced its own regulations on fire-resistant hydraulic fluids in December 1959, which were largely developed around water-in-oil emulsions, while some other countries adopted the National Coal Board requirements. The relatively appearance of this class of material as a fire-resistant hydraulic fluid arises from a market interest in products that are environmentally acceptable, and its presence is again due to the fact that some of these fluids have met the latest Factory Mutual flammability requirements. Vegetable oils are almost exclusively composed of triglycerides, which are esters of glycerol and fatty acids.