A study is made of the interaction of a random field of electromagnetic radiation with the free electrons of a plasma, with applications to astrophysical problems, in particular the theory of how thermodynamic equilibrium of the radiation in a hot universe is established. A kinetic equation describes the variation of the spectrum; special attention is devoted to induced scattering and the classical interpretation of induced energy and momentum transfer. In the spectra of radio sources with a high luminosity temperature, induced scattering can lead to a Bose condensation of photons, a shock wave and solitons. The scattering of strong low-frequency waves is considered in connection with their effect on pulsars and laboratory coherent generators.