Trichoderma reesei degrades native cellulose utilizing a set of cellulolytic enzymes dominated by two cellobiohydrolases, CBHI and CBHII. These enzymes exhibit the typical two domain architecture of fungal cellulases, and both act primarily as exoglucanases liberating cellobiose from the ends of the polymeric cellulose chains. The three dimensional structures of the catalytic domains of CBHI and CBHII revealed that their active sites are situated in tunnels formed by long loops on the enzymes surface. The active sites of homologous endoglucanases lack these loops and have more open active sites permitting catalytic activity in the internal positions of cellulose chains. Site-directed mutagenesis and structural studies have identified the key catalytic residues of both CBHI and CBHII. Similarly, the primary interaction surface of the cellulose-binding domain has been defined and residues responsible for its tight binding to cellulose identified.