Heat transfer fluids are used in indirect process heating and cooling systems to transfer heat into the process or remove heat from the process. In some applications, the required heating temperature is high enough that the operating pressure of a steam system would be impractically high. For these applications, a heat transfer fluid system can be used. Heat transfer fluid systems can operate at high temperature at relatively low pressure. Some heat transfer fluids can be used to provide cooling at temperatures too low for water systems. In some processes, the product must be heated during part of the manufacturing process and cooled during another part. A single heat transfer fluid system containing a heater and a cooler could be used for this type of application. Examples of high temperature heat transfer fluids include: molten salts, petroleum based mineral oils (these are often referred to as hot oils) and various synthetic fluids such as silicones, alkylated aromatics, terphenyls, and mixtures of diphenyls and their oxides (these are often referred to as thermal fluids). The focus of this chapter is on mineral oil and synthetic heat transfer fluids. This chapter discusses the heat transfer coefficient as a fluid characterization parameter, fluid chemistry and its impact on fluid properties, and test procedures used in the selection and maintenance of mineral oil heat transfer fluids. Heat transfer fluid system design, operation, and maintenance are also discussed.