α-Glucosidase inhibitors are marketed as therapeutic drugs for diabetes that act through the inhibition of carbohydrate metabolism. Inhibitors of the α-glucosidases that are involved in the biosynthesis of N-linked oligosaccharide chains have been reported to have antitumor, antiviral, and apoptosis-inducing activities, and some have been used clinically. α-Glucosidase inhibitors have interesting biological activities, and their design, synthesis, and screening are being actively performed. In quite a few reports, however, α-glucosidases with different origins than the target α-glucosidases, have been used to evaluate inhibitory activities. There might be confusion regarding the naming of α-glucosidases. For example, the term α-glucosidase is sometimes used as a generic name for α-glucoside hydrolases. Moreover, IUBMB recommends the use of “α-glucosidase” (EC 188.8.131.52) for exo-α-1,4-glucosidases, which are further classified into four families based on amino acid sequence similarities. Accordingly, substrate specificity and susceptibility to inhibitors varies markedly among enzymes in the IUBMB α-glucosidases. The design and screening of inhibitors without consideration of these differences is not efficient. For the development of a practical inhibitor that is operational in cells, HTS using the target α-glucosidase and the computer-aided design of inhibitors based on enzymatic information concerning the same α-glucosidase are essential.