The three-dimensional structures of various types of α-amylase family enzymes are briefly reviewed focused on their substrate specificities. Through these studies, it was shown that a conserved network structure composed of amino acids and a water exists at the active site of the enzymes of this family . The roles of the three essential catalytic residues are discussed based on the substrate-complexed structures of mutant enzymes. Particularly, the role of Asp297 (in TAA), which has not been clarified before, is discussed and shown to work crucially for evoking a distortion on the glucose ring at subsite -1, which leads to cleavage of the glucosidic bond. A reaction scheme involving the three essential catalytic residues is presented. Furthermore, the characteristic binding mode of the substrate amylose with respect to the (β/α)₈-barrel in the family enzymes is addressed.