(+)-L-Alliin is a non-proteinogenic, sulfur-containing amino acid derivative, specifically an S-alk(en)yl-L-cysteine sulfoxide. It is the primary precursor to allicin, the bioactive compound responsible for the characteristic odor and many of the pharmacological properties of garlic (*Allium sativum*). Chemically, it is the (+)-S-allyl-L-cysteine sulfoxide. This compound is a key natural product and a major storage form of sulfur in intact garlic. Upon tissue damage (e.g., cutting or crushing), alliin is rapidly hydrolyzed by the enzyme alliinase to produce allicin, pyruvate, and ammonia. Allicin is a potent antimicrobial, antioxidant, and anti-inflammatory agent. Consequently, (+)-L-Alliin itself is of significant interest in nutritional, pharmacological, and biochemical research focused on the health benefits of garlic, including studies on cardiovascular health, cancer chemoprevention, and antimicrobial activity. It is commercially available as a high-purity standard for analytical purposes (e.g., HPLC quantification in garlic extracts) and as a research biochemical for investigating the biosynthesis and metabolism of Allium compounds. Its stability in intact tissues makes it a crucial marker for garlic quality and standardization in dietary supplements.