A novel classification scheme for glycoside hydrolases was introduced in 1991 to overcome some of the limitations of the classical EC number system. The new system, based on amino acid sequence similarities, was subsequently extended to other carbohydrate-active enzymes such as the glycosyltransferases and the polysaccharide lyases. Here we examine the current status of this classification scheme which provides a unified framework for the analysis of the structures and mechanisms of carbohydrate-active enzymes in the genomic era.