Sequence-based classifications are one of the most powerful predictive methods for suggesting function for newly sequenced enzymes for which function has not been biochemically demonstrated. A classification system for glycosyl hydrolases, based on sequence similarity, has led to the definition of more than 100 different families. This classification is available on the CAZy (CArbohydrate-Active EnZymes) web site. The database provides a series of regularly updated sequence based classification that allow reliable prediction of mechanism (retaining/inverting), active site residues and possible substrates.  The online database is supported by CAZypedia, an online encyclopedia of carbohydrate active enzymes. Based on three-dimensional structural similarities, the sequence-based families have been classified into clans of related structure. Recent progress in glycosidase sequence analysis and 3D structure comparison has allowed the proposal of an extended hierarchical classification of the glycoside hydrolases.