This chapter discusses heat removal system materials: the coolant materials. The energy transferred to the coolant, as it flows past the fuel elements, is stored in the form of temperature and pressure, and is called the enthalpy of the coolant. The fluid medium, or coolant, is basically of two types: liquid and gaseous. The primary coolant gets fouled with impurities through wear and corrosion, and possibly through leaks in some fuel pins. The recommendation for the use of a gaseous substance as a reactor coolant can be made on the basis of general radiation stability, ease of handling, and absence of hazardous conditions. The liquid metals are well known for their commendable role as reactor coolants. Their special place as coolants is for reactors with high thermal fluxes, operating at high temperatures. Compatibility must be considered in the assessment of the capability of a coolant to perform the function of heat removal from a high power density core.