Enzymatic hydrolysis of cellulose by microorganisms is a keystep in the global carbon cycle. Despite its abundance only asmall percentage of microorganisms can degrade cellulose,probably because it is present in recalcitrant cell walls. Thereare at least five distinct mechanisms used by differentmicroorganisms to degrade cellulose all of which involvecellulases. Cellulolytic organisms and cellulases are extremelydiverse possibly because their natural substrates, plant cellwalls, are very diverse. At this time the microbial ecology ofcellulose degradation in any environment is still not clearlyunderstood even though there is a great deal of informationavailable about the bovine rumen. Two major problems thatlimit our understanding of this area are the vast diversity oforganisms present in most cellulose degrading environmentsand the inability to culture most of them.