A class of enzymes, each of which cleaves (i.e., cuts) oligosaccharides (e.g., the side chains on glycoprotein molecules) at a specific location within the chain. They are an important tool in carbohydrate engineering, enabling the carbohydrate engineer to sequence (i.e., determine the structure of) existing oligosaccharides, to create different oligosaccharides, and to create different glycoproteins via removal/addition/change of the oligosaccharide chains on glycoprotein molecules.