Abstract <p>Alginates are anionic unbranched plant and bacterial polysaccharides composed of mannuronic and guluronic acid residues. Alginates can form hydrogels under mild conditions in the presence of divalent cations (e.g., Ca²⁺). Because of their capacity to form gels, high biocompatibility, and relatively low cost, these polysaccharides are employed in pharmaceutical industry, medicine, food industry, cosmetology, and agriculture. Alginate oligomers produced by enzymatic cleavage of high-molecular-weight algal alginates are used as medicinal agents and dietary supplements. The global market for alginate-based products exceeds $1 billion. Alginates and their oligomers have attracted a special interest in biomedical sciences due to manifestation of various types of therapeutic activity. Across more than 50-year history of studies of alginates, over 60% scientific articles in this field have been published in the last 5 years. Unfortunately, the works dedicated to the mechanisms of biological activity of alginates and their oligosaccharides are still very scarce. This review analyzes the current state of research on the mechanisms (mainly biochemical) underlying biological and therapeutic activities of alginates (antioxidant, antibacterial, anti-inflammatory, antitumor, neuroprotective, antihypertensive, regenerative, and prebiotic). A comprehensive understanding of these mechanisms will not only improve the efficiency of alginate application in medicine and other traditional fields (cosmetology, food industry), but might also reveal their potential in new areas such as tissue engineering, nanobiotechnology, and bioelectronics.</p>