Electrons and holes in semiconductors recombine either radiatively, i. e. accompanied by the emission of a photon, or non-radiatively. In light-emitting devices, the former is clearly the preferred process. However, non-radiative recombination can, under practical conditions, never be reduced to zero. Thus, there is competition between radiative and non-radiative recombination. Maximization of the radiative process and minimization of the non-radiative process can be attained in a number of ways which will be discussed below.