The sourcing and characterization are of primary importance before considering the exploitation of seaweeds alginates by industry. The main species exploited for alginate production have alginate content up to 20-40% of their dry weight. Various grades of seaweed alginates are on the market and classified depending on their distribution pattern of M- and G-blocks, molecular weight, purity and composition. This chapter discusses the developments on structural and physicochemical characterizations of alginates as well as their biological properties. The structural characterization of alginate includes elementary biochemical assays but also more complex analyses leading to fine structure identification. The global alginate market is mainly driven by the food applications where the gelifying properties of these seaweed polysaccharides authorized as additives by major regulatory agencies including the European Commission and Food and Drug Administration are valorized.