Although silicon is an indirect semiconductor, light emission from silicon is governed by the same generalized Planck's radiation law as the emission from direct semiconductors. The emission intensity is given by the absorptance of the volume in which there is a difference of the quasi Fermi energies. A difference of the Fermi energies may result from the absorption of external light (photoluminescence) or from the injection of electrons and holes via selective contacts (electroluminescence). The quantum efficiency may be larger than 0.5 for carrier densities below 10^{15} cm-3. At larger densities, non-radiative recombination, in particular Auger recombination dominates. At all carrier densities, the relation between emission intensity and difference of the quasi Fermi energies is maintained. Since this difference is equal to the voltage of a properly designed solar cell, luminescence is the key indicator of material quality for solar cells.