Alginate is a natural, anionic polysaccharide typically obtained from brown seaweed but it can be synthesized by some bacteria. It is a water-soluble, linear copolymer consisting of two kinds of alternating blocks, guluronic acid (G block) and mannuronic acid (M block) units, that are irregularly arranged in GG, MG, and MM blocks, while the ratio and pattern of guluronate to mannuronate blocks varies depending on the natural source of alginate. Most favorable properties of alginate are its biocompatibility, low toxicity, relatively low cost, and mild and easy gelation. Alginates in the presence of divalent cations (mostly calcium cations) very quickly form a hydrogel, polymeric networks with three-dimensional configuration, capable of imbibing high amounts of water. This book discusses the chemical structure, the uses, and health benefits of alginic acid.