Cellulase refers to a class of enzymes produced chiefly by fungi, bacteria, and protozoans that catalyze the cellulolysis (or hydrolysis) of cellulose. However, there are also cellulases produced by other types of organisms such as plants and animals. Several different kinds of cellulases are known, which differ structurally and mechanistically. The EC number for this group of enzymes is EC 3.2.1.4.

Five general types of cellulases based on the type of reaction catalyzed:

Endo-cellulase breaks internal bonds to disrupt the crystalline structure of cellulose and expose individual cellulose polysaccharide chains

Exo-cellulase cleaves 2-4 units from the ends of the exposed chains produced by endocellulase, resulting in the tetrasaccharides or disaccharide such as cellobiose. There are two main types of exo-cellulases (or cellobiohydrolases, abbreviate CBH) - one type working processively from the reducing end, and one type working processively from the non-reducing end of cellulose.

Cellobiase or beta-glucosidase hydrolyses the exo-cellulase product into individual monosaccharides.

Oxidative cellulases that depolymerize cellulose by radical reactions, as for instance cellobiose dehydrogenase (acceptor).

Cellulose phosphorylases that depolymerize cellulose using phosphates instead of water.