Chitosan is one of the most abundant polysaccharides in nature. It consists of β-1, 4-linked D-glucosamine (GlcN) and N-acetyl-β-Dglucosamine (GlcNAc) residues. Chitosanases catalyze the hydrolytic breaking of its glucosidic bond and produce chitosan, which are widely applied for various purposes in medicine, agriculture, food and industry. Chitosanases found in microorganisms and some plants have stable physical and chemical properties. They are classified into three subclasses based on the substrate specificity and the cleavage positions of the partially acetylated chitosan. According to the amino acid alignment, most sequenced chitosanases belong to three-glycoside hydrolase families designated as number 46, 75 and 80. The three dimensional structures have been determined only for the chitosanases from Streptomycse sp. N174 and Bacillus circulans MH-K1.