Sialidases (EC.3.2.1.18) are an important class of glycosidases that are widely found in animals and microorganisms. These enzymes catalyze the cleavage of the terminal sialic acid from various oligosaccharides and sialoglycoconjugates, and play important roles in diverse biological processes such as lysosomal catabolism, tumorigenesis and microbial pathogenesis. Generally, sialidases cleave glycosidic linkages. Under appropriate reaction conditions in vitro, however, the enzymes can catalyze the formation of the glycosidic linkages by transglycosylation reaction. This synthesis activity is important for the large acquisition of sialosides, which would be helpful to promote the basic research on their functions as well as their applications in food and pharmaceutical industries. This paper reviews the structure and catalytic mechanism of sialidases, physiological function, transglycosylation and their application in oligosaccharide synthesis.