Cellulose is an important industrial raw material and a source of renewable energy. The crystalline structure of cellulose is biodegradable by the combined activities of many different enzymes. Cellobiohydrolases, the key cellulases, have extended tunnel-shaped active sites. Owing to the design of their active-site tunnels, they do not make cuts within cellulose chains but release cellobiose from the chain ends. Cellobiohydrolases may therefore prove to be useful for achieving subtle modifications of fiber properties in paper or textiles without the loss of fiber strength commonly observed with other cellulases.