Human milk oligosaccharides (HMOs) are a diverse group of glycans unique to human milk, first identified as a prebiotic "bifidus factor" that promotes beneficial gut bacteria. Recent research suggests HMOs have broader roles, including antiadhesive antimicrobial properties, modulating immune responses, and providing sialic acid for brain development. HMOs are structurally complex, with variations based on the mother's blood group and lactation stage. They are composed of glucose, galactose, N-acetylglucosamine, fucose, and sialic acid, with sialic acid being the predominant form. HMO biosynthesis begins with lactose and is extended by adding disaccharides, leading to branched or linear structures. HMOs are absorbed in the infant's intestine and reach the systemic circulation, with some being excreted in urine. HMOs may benefit the breast-fed infant by promoting beneficial bacteria, preventing pathogen attachment, modulating immune responses, and providing essential nutrients. However, most evidence comes from in vitro or animal studies, and more research is needed to confirm their effects in human neonates. HMOs also show potential protective effects against infections, including necrotizing enterocolitis, and may contribute to brain development through sialic acid. While HMOs are primarily studied for their benefits to infants, they may also influence the mother's health, such as by modulating the bacterial composition in milk. HMOs are structurally and functionally distinct from oligosaccharides in other mammals, with humans having the highest diversity and concentration. Further research is needed to fully understand HMOs' roles and potential applications.Human milk oligosaccharides (HMOs) are a diverse group of glycans unique to human milk, first identified as a prebiotic "bifidus factor" that promotes beneficial gut bacteria. Recent research suggests HMOs have broader roles, including antiadhesive antimicrobial properties, modulating immune responses, and providing sialic acid for brain development. HMOs are structurally complex, with variations based on the mother's blood group and lactation stage. They are composed of glucose, galactose, N-acetylglucosamine, fucose, and sialic acid, with sialic acid being the predominant form. HMO biosynthesis begins with lactose and is extended by adding disaccharides, leading to branched or linear structures. HMOs are absorbed in the infant's intestine and reach the systemic circulation, with some being excreted in urine. HMOs may benefit the breast-fed infant by promoting beneficial bacteria, preventing pathogen attachment, modulating immune responses, and providing essential nutrients. However, most evidence comes from in vitro or animal studies, and more research is needed to confirm their effects in human neonates. HMOs also show potential protective effects against infections, including necrotizing enterocolitis, and may contribute to brain development through sialic acid. While HMOs are primarily studied for their benefits to infants, they may also influence the mother's health, such as by modulating the bacterial composition in milk. HMOs are structurally and functionally distinct from oligosaccharides in other mammals, with humans having the highest diversity and concentration. Further research is needed to fully understand HMOs' roles and potential applications.