A relevant and modern objective for second-millennium biotechnology is the enzymatic conversion of renewable cellulosic biomass to inexpensive fermentable sugars. New and more efficient fermentation processes will convert this biological “currency” to a variety of commodity products. Although early strides will be made using process development and engineering disciplines, mid-term and longer advances must rely heavily on insight gained through protein and metabolic engineering technologies. These challenging goals can be met most effectively by the full integration of academic, federal, and industrial efforts in teams that develop and apply new fundamental knowledge to key cost drivers. The present chapter describes a comprehensive and illustrative analysis on basic and applied aspects of fungal cellulases. A description of different substrates of cellulases as well as fundamental biotechnological and catalytic aspects are reviewed and discussed to illustrate the potential of cellulases in the food and bioprocess industries. Moreover, an emphasis on the scientific and technological advances and challenges of cellulase study is also described.