Equilibrium in a crystal containing nearly non-interacting electrons exists when the electrons are distributed according to the Fermi-Dirac function and the phonons and photons distributed according to the Bose-Einstein function. There are thus four general ways in which non-equilibrium may be brought about. In a “quasi-equilibrium” distribution, relative occupancies among energy levels of given sets (e.g. a given band) have the same dependence on energy as in equilibrium. Then the determination of one parameter per band, in addition to the electrostatic potential, all as functions of position and time, serve to solve the problem. When quasi-equilibrium does not pertain, kinetic equations are needed. The principles of microscopic reversibility and detailed balance are often helpful in reducing the number of independent transition probabilities required.