Cellulase refers to a class of enzymes produced chiefly by fungi, bacteria, and protozoans that catalyse the cellulolysis (or hydrolysis) of cellulose. The enormous potential that cellulases have in biotechnology is the driving force for continuous basic and applied research on these biocatalysts from fungi and bacteria. Cellulases are found in many fields, such as animal feeding, brewery and wine, food, textile and laundry, pulp and paper products. The growing interest toward the conversion of lignocellulosic biomass into fermentable sugars has generated an additional request for cellulases and their related enzymes. This book presents research in the study of cellulase, including biotechnological applications of microbial cellulases; using agro-industrial by-products as raw material for cellulase production; and the enzyme saccharification of cereal crop residues using dilute alkali pre-treatment.