Tannins are a heterogeneous class of high-molecular-weight polyphenolic secondary metabolites produced by plants. They are characterized by multiple hydroxyl groups and aromatic rings that enable strong binding to proteins, alkaloids, and polysaccharides. This astringency is exploited in food (tea, wine, fruits), leather tanning, and traditional medicine. Tannins exhibit antioxidant, antimicrobial, and anti-inflammatory activities but can reduce nutrient bioavailability by inhibiting digestive enzymes and forming insoluble complexes with minerals and proteins. They occur as hydrolyzable (e.g., gallotannins) or condensed (proanthocyanidins) forms. In food science they contribute to flavor, color stability, and shelf life. Environmental persistence varies; they are biodegradable yet can exert aquatic toxicity at high concentrations through oxygen depletion and pH effects.