Cellulose hydrolysis requires enzymatic cleavage of p-1,4-glycosidic bonds between D-glucose units. GHs with this function are generally called cellulases, and can be divided into three classes based on their enzymatic activities (Lynd et al. 2002). The known classes of cellulose­degrading enzymes are summarized in Table 3 and illustrated in Fig. 6. Endoglucanases randomly cleave interior glycosidic bonds in cellulose, releasing oligosaccharides of varied lengths with new reducing and non­reducing ends. This function greatly contributes to solubilizing the cellulose polymer by reducing molecular size and creating accessible chain ends for further attack. Cel48F, CelC, Cel7B proteins are typical endoglucanses critical in cellulose degradation in Clostridium cellulolyticum (Perret et al. 2004), C. thermocellum (Wang et al. 1993) and Trichoderma reesei (Kleman-Leyer et al.1996), respectively. RNAi-based repression of Ce148F C. cellulolyticum resulted in a 30% decrease in the activity of the cellulolytic system on microcrystalline cellulose (Perret et al. 2004). In contrast, exoglucanases act from chain ends of cellulose oligosaccharides to processively chip off glucose or cellobiose (di-glucose) units (Lynd et al. 2002). Glucose- and cellobiose-releasing exoglucanases are also called exo-1,4-p-glucosidases and cellobiohydrolases, respectively. p-glucosidases typically split cellobiose dimers, or sometimes cellotrioses, into individual glucose units, thereby releasing the inhibitory effect of accumulated cellobiose on exo — and endo-glucanases (Gruno et al. 2004; Yue et al. 2004). These three classes of cellulases are critical to cellulose degradation and have been applied in different industries (Kuhad et al. 2011). In addition, some bacteria, like C. stercorarium and C. thermocellum, also encode other enzymes that act in cellulose degradation. Cellobiose phosphorylases are able to phosphorylate cellobiose to produce one glucose molecule and another activated glucose- 1-phosphate molecule without using ATP (Alexander 1968; Reichenbecher et al. 1997). Recently, a cellobiose dehydrogenase from Neurospora crassa was found to enhance cellulose degradation by coupling the oxidation of cellobiose to the reductive activation of a copper-dependent polysaccharide monooxygenase (Sygmund et al. 2012). Cellulolytic microorganisms produce a diversity of these enzymes for synergistic catalysis to significantly accelerate cellulose degradation (Doi 2008; Fontes et al. 2010).