Similar to gaseous materials, which consists of numberless particles, radiation also consists of numberless photons. Hence, in the viewpoint of statistical dynamics, radiation is similar to fluid in some sense. Indeed, the fundamental equation and its moment equations are parallel for radiation and matter, except that photons are massless particles. One of the essential difference between radiation and matter is the speed of ‘particles’ and therefore, the interaction distance is remarkably different. Gaseous particles can travel at sub-light speed, and in usual gaseous objects, the gas pressure is isotropic and local thermodynamical equilibrium is quickly established, and the fluid approximation holds. In contrast, radiation in the free space travels at the speed of light, and therefore, can interact gaseous particles at great distance. As a result, we sometimes consider an anisotropic radiation field, and non-local interaction. Another essential difference is that the fluid particles are fermions, but photons are bosons; photons are produced and destroyed by gaseous matter, and therefore, the numbers of photons change, while gaseous particles do not, except for the electron-positron pair plasma in the relativistic regime. As a result, we must consider emission, absorption, and scattering of radiation by gaseous matter in the radiation hydrodynamical situations. In this chapter, we first summarize the basic concepts in radiation hydrodynamics; mean free path and optical depth, photon creations (emission) and destructions (absorption) and scattering. We further review the concepts of the local thermodynamical equilibrium (LTE) and blackbody radiation. We then show the radiation force and the Eddington luminosity in relating to the momentum exchange between radiation and matter. We also briefly describe radiative viscosity and heat conduction. Finally, we introduce the adiabatic exponents of radiative fluids.