Heparins used in therapy are largely constituted by sequences of the trisulfated disaccharide <i>L</i>-iduronic acid-2-sulfate→<i>D</i>-glucosamine-N,6-disulfate. These regular sequences are interrupted by undersulfated (occasionally, oversulfated) sequences containing <i>D</i>-glucuronic acid and N-acetylated <i>D</i>-glucosamine. Different heparin sequences are binding domains for heparin cofactors and plasma proteins. The active site for antithrombin is a specific pentasaccharide sequence containing 3-O-sulfated <i>D</i>-glucosamine. Heparin cofactor-II binds, less specifically, mostly to the regular sequences. The conformational flexibility of iduronic acid residues contributes to the binding versatility and to the ‘biological reactivity’ of heparin.