Significant conversion of recalcitrant cellulosic biomass to fermentable sugars requires, at a minimum, endo-glucanase (EC 3.2.1.4) and exo-glucanase (EC 3.2.1.91) activities to reduce the insoluble cellulose chains to 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 cost associated with vast quantity of cellulase required. At ~50mg enzyme/gram cellulose, the current level of microbially-expressed enzymes 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 to produce and deliver the enzymes offers a convenient and cost effective alternative. Syngenta has pioneered the concept of in planta expression of enzymes and traits focused on the biofuels industry. Indeed, Syngenta is the only company to date to have taken a plant-expressed biofuels trait, our corn-expressed amylase for dry grind ethanol production, into the US regulatory system. We have successfully generated transgenic crop plants, including maize and tobacco, expressing active bacterial and fungal cellulases at high levels. This paper will outline the characterization of in planta-expressed microbial cellobiohydrolases (CBH I and II) and endo-glucanases and will demonstrate the capacity of these enzymes to function in defined enzyme cocktails for the degradation of lignocellulosic biomass.