Technical Abstract: Consolidated bioprocessing (CBP), in which anaerobic bacteria produce their own cellulolytic enzymes and ferment the products of cellulose hydrolysis to ethanol in a single reactor, is regarded as a promising future route to cellulosic ethanol. Some of the current limitations to practical use of this platform (some of which are also common to simultaneous saccharification processes using enzyme/yeast mixtures) have been successfully addressed in another natural cellulose-degrading system, namely the ruminal fermentation. We examine here some of the key process breakthroughs developed by cattle and other ruminant animals that can guide future research in improving engineered CBP systems. These include, among others, an elegant and effective physical pretreatment; operation at high solids loading under non-aseptic conditions; minimal nutrient requirements beyond the plant biomass itself; efficient fermentation of nearly all plant components; efficient recovery of primary fermentation end products; and production of useful co-products. Although the ruminal fermentation does not produce ethanol, operation of the ruminal fermentation as a dedicated industrial process is feasible if the abundant, energetic fermentation products (primarily volatile fatty acids and methane) can be economically recovered and converted to other organic compounds, particularly transportation fuels.