Cellulase, multi-component refers to class glycosyl hydrolases (GHs) that contain three major types of enzymatic activities are found: (i) endoglucanases (ii) exoglucanase and (iii) β-glucosidases (EC 188.8.131.52). Cellulases are the enzymes which hydrolysis cellulosic biomass and are being produced by the microorganisms grown over cellulosic matters. Cellulase is an important enzyme which can be obtained from cheap agro wastes by using submerged fermentation and solid state fermentation. Cellulases have attracted much interest because of the diversity of their industrial application. This book will be discussing about cellulose in agricultural wastes, cellulases and its types, cellulolytic microorganisms, cellulase production, Multiplicity and improvement, regulation, its purification & characterization and application of cellulases.