Sulfated galactans (SGs) are highly anionic marine galactose-composed homopolysaccharides. Although their structures vary among species, their main features are conserved among phyla. Green algal SGs are quite heterogeneous, although preponderantly composed of 3-beta-D-Galp units. The red algal SGs (like agar and carrageen) are composed of repeating disaccharide units with different sulfation patterns which vary among species. The SGs from invertebrates such as sea urchins and ascidians (tunicates), and from the unique description of a sea-grass, are composed of well-defined repetitive units. Chains of 3-linked beta-galactoses are highly conserved in some marine taxonomic groups, with a strong tendency toward 4-sulfation in algae and marine angiosperm, and 2-sulfation in invertebrates. These carbohydrates are extracellular components of the cell wall in plants, of the body wall in tunicates, and of the jelly coat in sea urchin eggs. In sea urchins, the SGs are also responsible to induce the acrosome reaction. However, the wide range of potential pharmacological uses, especially as anticoagulants and antithrombotics, is the main reason for the increasing interest in these sugars. Both natural and clinical actions of SGs have a direct relation to their structural features, since the intermolecular complexes between SG and target proteins are much more stereospecific than only electric charge-dependent. This review will present an overview about the principle structural and functional information of SGs. Other important aspects concerning occurrence, biology, phylogeny, and future directions, will also be reported.