The major physical phenomena associated with the gradual degradation of electroluminescent diode efficiency (both in the stimulated and spontaneous emission modes) are reviewed. The degradation is mainly associated with an increase in the density of nonradiative centers in the vicinity of the p-n junction. Both isolated and clustered centers (which contribute to increased absorption) are formed and the process of degradation is strongly dependent on metallurgical factors such as dislocations, mechanical stresses and contaminants. No evidence exists of inherent limitations on the diode life apart from metallurgical factors. Many thousands of hours of useful operating life have been obtained from devices fabricated with state-of-the-art technology