Sialidases or neuraminidases are enzymes that catalyze the cleavage of terminal sialic acids from oligosaccharides and glycoconjugates. They play important roles in regulate metabolism of sialic acid-containing molecules in biological systems. They are also virulence factors for many viruses and pathogenic bacteria. We are interested in profiling substrate specificities of sialidases regarding sialyl linkage, sialic acid forms, and internal glycans to better understand the functions of this class of enzymes. In addition, designing selective inhibitors against sialidases from different sources is another strategy to help us understand the roles of sialidases which may also lead to the development of selective sialidase inhibitors as potential therapeutics.