Crystalline cellulose is the most recalcitrant component of fermentable biomass. The structure and shape of microcrystalline cellulose fibrils have been studied with molecular mechanics simulations. These studies provide insight into features of the substrate which may be more accessible to enzymatic hydrolysis, and provide a basis for further studies of cellulase-cellulose interactions. Differences in crystal packing and crystal shapes as they relate to biological samples will be described.