The conversion of biomass to energy is now more promising due to recent advances in the enzymatic hydrolysis of cellulose with subsequent conversion to ethanol. One factor in achieving economic viability will be processing the biomass at high solids concentration, so that the energy required for heating and other processing steps can be minimized, and the concentration of ethanol produced can be increased. However, processing at high concentrations creates challenges for pumping and mixing. While reducing particle size can improve the rheology, with added benefits for reaction kinetics, particle size reduction incurs additional costs. It is therefore important to understand the rheological and mass transfer properties of these slurries in order to design optimal, efficient processes. In this presentation, we report experiments employing various lignocellulosic materials, such as wood pulp and corn stover. Methods for measuring and controlling the particle size distributions will be discussed. Rheological measurements employ torque rheometry, a method commonly employed in the food and polymer industries. We describe the rheology of biomass slurries and its dependence on such variables as solids concentration, deformation rate, particle size and size distribution, and temperature.