This review reconsiders the concept of the alpha-amylase family in the light of the recent wealth of information on the structures, the catalytic mechanisms, and the classification of amylases. We proposed a general concept for an enzyme family, the alpha-amylase family including most of the amylases and related enzymes in 1992, based on the structural similarity and the common catalytic mechanisms. The study on neopullulanase was the key to open the door for the formulation of the concept. We discovered a new enzyme, neopullulanase, and proved that the enzyme catalyzes both hydrolysis and transglycosylation at alpha-1,4- and alpha-1,6-glucosidic linkages by one active center. Results from a series of experiments using neopullulanase indicated that the four reactions mentioned above could be catalyzed in the same mechanism. Progress in X-ray crystallographic analysis has allowed researchers to observe the structural similarities among alpha-amylases, cyclodextrin glucanotransferases, and an isoamylase. The primary structural analyses and the secondary structural predictions also suggest a close relationship among enzymes with three-dimensional structures which catalyze one of the four reactions. They possess a catalytic (beta/alpha)(8)-barrel as observed in the crystal structure of alpha-amylases, cyclodextrin glucanotransferases, and an isoamylase. Two crucial points, the common catalytic mechanisms and the structural similarities among the enzymes which catalyze the four reactions, led us to propose the concept of the alpha-amylase family. We would like to point out the significance and problems of the sequence-based classification of glycosyl hydrolases. The possible catalytic mechanism of the alpha-amylase family enzyme is also described for the rational design of tailor-made artificial enzymes.