Cellulases are a complex group of enzymes which are secreted by a broad range of microorganisms including fungi, bacteria, and actinomycetes. In the natural environment, synergistic interactions among cellulolytic microorganisms play an important role in the hydrolysis of lignocellulosic polymer materials. In fact, it is the combined action of three major enzymes which determines the efficiency of this process. They are exoglucanases, endoglucanases, and β-glucosidase. Microorganisms produce these enzymes in a diverse nature which determines their efficiency in cellulose hydrolysis. During the cellulose degradation reaction, the enzyme targets the β-1, 4-linkages in its polymeric structure. This is an essential ecological process as it recycles cellulose in the biosphere. The application of this same scenario for industrial purposes is identified as an emerging area of research. Biofuel production, textile polishing and finishing, paper and pulp industry, and lifestyle agriculture are among the key areas where cellulase enzyme shows a broader potential. The objective of this chapter is to discuss the structure, function, possible applications, as well as novel biotechnological trends of cellulase enzymes. Furthermore, possible low-cost, enzymatic pretreatment methods of lignocellulosic material in order to use it as an efficient raw material for biofuel production will be discussed.