Gaseous coolants have the great advantage of having a well-defined phase state. Unlike liquid coolants, they are not subject to a change of phase, with the resulting complicated two-phase flow problems during abnormal operating conditions. However, they have the disadvantages of a low heat capacity and low heat transfer coefficients, the latter necessitating heat transfer enhancement or low operating temperatures. A wide variety of gases have been considered for nuclear reactor cooling, but only those that have been used or have had se­rious evaluation will be discussed here.