Significant degradation of recalcitrant cellulosic biomass to fermentable glucose requires, at a minimum, endoglucanase (EC 3.2.1.4) and exoglucanase (EC 3.2.1.91) activities to reduce the insoluble cellulose chains to (primarily) soluble cellobiose units which are subsequently hydrolyzed to glucose by β-glucosidase (EC 3.2.1.21). A major obstacle to the development of a commercially viable cellulosic ethanol industry is the costs associated with vast quantity of cellulase required. At ~50mg enzyme/gram cellulose, the current level of microbially-expressed enzyme required for efficient degradation of lignocellulosic biomass makes microbial expression an economically untenable means for enzyme production. While efforts to address this problem have primarily focused upon engineering more efficient cellulases and maximizing fungal expression, utilizing plants as the enzyme factory offers a convenient and low cost source of the enzymes. We have successfully generated transgenic crop plants, including maize and tobacco, expressing active bacterial and fungal cellulases at high levels. The results presented here demonstrate the capacity of plant-expressed Trichoderma reesei cellobiohydrolase I (CBH I) to function in a defined enzyme cocktail for the degradation of pretreated lignocellulosic biomass. These data illustrate the potential of in-planta cellulase production as a means to significantly decrease (if not nearly eliminate) the costs of enzyme production for the biofuels industry.