Living organisms produce a wide variety of polymeric carbohydrates, of which cellulose is the most abundant. The main deterioration of cellulose is caused by microorganisms, and the biological degradation of cellulosic substances is an extremely important process for the maintenance of the carbon balance in nature. Since crystalline cellulose is an insoluble substance with a supramolecular structure, the enzymological approaches to this substrate are rather complicated and difficult . In most cellulolytic microorganisms, several cellulase components together constitute a "cellulase system" and cellulosic materials are converted to soluble sugars by their synergistic action . To understand completely the biodegradation mechanism of cellulose, therefore, the strict purification and characterization of each cellulase component is absolutely necessary . This review describes the cellulolytic enzymes and focuses on the mechanism of their action . It is divided into seven sections and includes the historical background of cellulase . Our recent findings concerning the enzyme mechanisms involved in native cellulose degradation are also described.