Thermal radiation is produced by particles with a Maxwellian distribution function, which are found in collisional plasmas, such as in the solar chromosphere or in flare loops. The main type of thermal radiation is free-free (bremsstrahlung), which can be observed in hard X-rays as well as in microwaves. The equilibrium between absorption and emission processes is expressed with the radiative transfer equation. A thermal equilibrium is realized in a black body according to Kirchhoff’s law. In a plasma, where electrons and ions are free (above the ionization temperature), free electrons become scattered in the Coulomb field of ambient free ions, which produces the so-called free-free bremsstrahlung emission. In a thermal plasma there are abundant collisions between free electrons and free ions. There are a large number of atomic processes that are possible in a collisional plasma, including various ionization and recombination processes. A local thermodynamic and ionization equilibrium defines the number of bound-bound transitions in a plasma, expressed by the Saha equation.