Sialic acids are a family of nine-carbon acidic sugars found at the nonreducing terminus of many glycoconjugates. Sialidases can remove these sugar units selectively from cell surfaces, membranes, or purified glycoconjugates. In this unit, sialidase digestion of purified glycoproteins is described as is treatment of intact cells. The physical properties of the four most useful sialidases are discussed along with their relative activities against sialic acids with different modifications and in different linkages.