Fermentation is an anaerobic process that occurs in the cytoplasm of cells.  In fermentation reactions, pyruvic acid produced by glycolysis either serves as a final electron and hydrogen acceptor from NAD forming lactic acid (lactic acid fermentation), or is decarboxylated with the resulting acetaldehyde to form alcohol (alcoholic fermentation).

Other products can be produced depending upon the species, but all generate a net gain of only the two ATP molecules formed during glycolysis.