Most plant cellulases belong to the glycosyl hydrolase family 9, which is both enzymatically and structurally well characterized in microbes. The microbial enzymes include both randomly acting endoglucanases active on soluble and amorphous substrates and processive endoglucanases, which can also degrade crystalline cellulose. The corresponding plant enzymes have been difficult to purify and express in heterologoushosts and thus their modes of action have remained obscure. Here we have taken a molecular modelling approach and describe some structural features characteristic for soluble and membrane-bound family 9 cellulases in plants and filamentous fungi.