Book excerpt: When the temperature of a gas is not too high and the density of a gas is not too low, the transfer of heat by radiation is usually negligibly small in comparison with that by conduction and convection. However, in the hypersonic flow of space flight, particularly in the re-entry of a space vehicle, and in the flow problem involving nuclear reaction such as in the blast wave of nuclear bomb or in the peaceful use of the controlled fusion reaction, the temperature of the gas may be very high and the density of the gas may be very low. As a result, thermal radiation becomes a very important mode of heat transfer. A complete analysis of such high temperature flow fields should be based upon a study of the gasdynamic field and the radiation field simultaneously. Hence during the last few years, considerable efforts have been made to study such interaction problems between gasdynamic field and radiation field and a new title, Radiation Gasdynamics, has been suggested for this subject. Even though radiative transfer has been studied for a long time by astro physicists, the interaction between the radiation field and the gadsynamic field has been only extensively studied recently.