Cellulases play a central role in the recycling of lignocellulose. Molecular biology is now helping to unravel the complex enzymology of cellulose hydrolysis and giving information that will be of use both for the improvement of existing processes and for the development of completely new areas. Some topics discussed are the following: cellulases, the cloning of cellulase genes, similarity in architecture between cellulases, the function of the terminal domains, the catalytic mechanism and molecular structure, secondary structures, the role of glycosylation in cellulase function, cellulase diversity, the evolution of cellulase genes, and the future.