Publisher Summary This chapter describes isolation, purification, and characterization of monomeric sialic acids. There are two basic procedures for liberating sialic acids from glycosidic linkages—enzymatic and acid hydrolysis. In the former case, a variety of sialidases may be used, which differ in specificity for sialic acid linkage or species. Quantitation of sialic acids usually requires purified samples because a number of substances are known to interfere with certain tests. Colorimetric sialic acid quantitation is still one of the most important methods to determine the amount of the sugars in a given sample accurately. Two basically different tests are used—the orcinol/Fe 3+ /HCl or resorcinol/Cu 2+ /HCl assay and the periodic acid/thiobarbituric acid test. The procedures for a microadaptation of the orcinol/Fe 3+ /HCl and the periodic acid/thiobarbituric acid assays are discussed. The determination of sialic acids in serum, urine, or tissues or on cells is often used in clinical applications, because it seems to be a valuable marker for certain malignancies.