In this chapter we consider important high-energy processes producing radiation in low density and high temperature gases with or without magnetic fields. In such gases the radiation fields resulting from interactions among photons, plasmas, and magnetic fields are much different from the blackbody ones. In the case where there are strong magnetic fields, the important radiation process is synchrotron one. If electron and/or photon energies are as high as the electron rest mass energy, the Compton and inverse Compton processes become important. Furthermore, the electron-positron pair processes appear in such a high-energy regime. The radiation spectra resulting from these high energy processes are of importance to know high energy phenomena in astrophysical objects. The essential parts of these high energy processes are briefly summarized.