Glycoside hydrolases (E.C 3.2.1.-) are enzymes that catalyze the cleavage of glycosidic bonds in oligosaccharides, complex sugars and various glycoconjugates. In this work structures of B. thetaiotaomicron β-1,4-mannosidase, B. halodurans β-1,4-xylosidase are used for describing and comparing their mechanistic and tertiary and quaternary structural basis for achieving efficient catalysis.