Publisher Summary This chapter presents the up‐to‐date information on the structure of chondroitin sulfate (CS) and dermatan sulfate (DS). CS is an anionic linear polysaccharide, which is synthesized as part of proteoglycan (PG) molecules in vertebrates and invertebrates. CS consists of alternating disaccharide units of glucuronic acid and galactosamine and is attached to serine (Ser) residues of the protein cores via a tetrasaccharide linkage. Despite the simplicity of the backbone structure, the CS molecule is complex enough to carry biologic information, and thus determine many biologic functions. A highly modified CS chain, at least at the level of uronic acid, is called “DS.” The complexity of CS/DS is brought about by structural features, such as sulfonylation at hydroxyl groups of galactosamine and glucuronic acid residues, the epimerization of certain glucuronic residues to iduronic acid, and the distribution of these uronic acid residues through the polymeric chain. Structural studies of CS chains from vertebrate and especially mammalian species have shown the great heterogeneity of structural organization.