Anaerobic Gram-negative and Gram-positive fermenting bacteria, for example bacteria of the genus Clostridium, perform stages of the anaerobic digestion of organic wastes such as the hydrolysis of biopolymers and fermentation of monomers. They also are often used to indicate fecal pollution in water. Anaerobic fermenting Archaea are prokaryotes that are distinctive from Gram-positive and Gram-negative bacteria in terms of their molecular biological properties. A tutorial with solutions and a quiz bank are added to this chapter.