In fermentative metabolism, organisms degrade organic substrates, such as carbohydrates, amino acids, organic acids, or purines and pyrimidines, in the absence or without the involvement of terminal electron acceptors. The reducing equivalents withdrawn from these substrates during breakdown are used for the reduction of internally formed metabolic intermediates, which are then secreted. Because the average energy gain in fermentations is low, the rate of substrate throughput is high and the substrates are converted into end products with an efficiency of often higher than 90%. The routes of substrate degradation are similar to those followed in respiratory metabolism, but the reductive branch that dissipates the electrons and reoxidizes the coenzymes is highly diverse and defines the classification of the fermentation type according to the end products secreted. Organisms are classified into the groups of obligatory and of facultative fermenters. Members of the facultative class of organisms, in contrast to those of the obligatory ones, are able to switch to a respiratory type of metabolism when they are provided with an external electron acceptor like oxygen or nitrate. This switch of metabolism is brought about by the complex interaction of several global regulators and by several specific control elements. Concomitant to the induction of the respiratory pathways, a repression of the fermentative route takes place. Many processes in industrial and food microbiology rely on the fermenting metabolism of microorganisms, and they are the subject of intensive research seeking to optimize rates, yields, and metabolic end product spectra.