The diversity of oligo- and polysaccharides provides an abundance of biological roles for these carbohydrates. The enzymes hydrolysing these compounds, the glycoside hydrolases, therefore mediate a wealth of biological functions. Glycoside hydrolases fall into a number of sequence-based families. The recent analysis of these families, coupled with the burgeoning number of 3D structures, provides a detailed insight into the structure, function and catalytic mechanism of these enzymes.