Recombination of electrons and holes may take place in the host crystal or at impurity centres, the energy being removed by radiation of a light quantum, by multiphonon emission, or by an Auger process. The probabilities for each of these six processes are discussed. While the lifetime in semiconductors is usually determined by multiphonon recombination at impurity centres, Auger recombination in the host crystal can be expected to dominate in small-band-gap crystals containing large concentrations of free carriers. Radiative recombination in the host crystal may limit the lifetime in semiconductors where band-to-band transitions are direct, provided that the specimens are reasonably free of recombination centres.