Human milk contains 17.7 g/L of human milk oligosaccharides (HMOs) in colostrum and 11.3 g/L in mature milk, accounting for 20% of the 7% of sugars it contains. These are complex mixtures of oligosaccharides that have a lactose unit at the reducing end, and about 200 chemical structures have been determined to date. When breastfed infants consume their mothers’ breast milk, most of the HMOs are not digested in the small intestine but reach the colon, where they are metabolized by bifidobacteria, which play an important role in maintaining health. The metabolic pathways of HMOs by some species of bifidobacteria have been largely elucidated over the past 15 years. HMOs have been shown to have functions such as infection defense against pathogenic microorganisms, immune regulation, enhancement of intestinal barrier function, prevention of necrotizing enterocolitis, and activation of brain nerve function, either through direct action on blood cells in the circulation process along with that on epithelial cells or through metabolic products by intestinal bacteria. On the other hand, cow’s milk, which is used as a raw material for infant formula, has a low concentration of milk oligosaccharides, and there have been technical problems in manufacturing it functionally closer to breast milk. In recent years, several types of HMOs (2′-FL, 3-FL, LNT, LNnT, DFL, 3′-SL, 6′-SL) have been industrially produced, and intervention trials in bottle-fed infants have been conducted. Infant formulas supplemented with 2′-FL and LNnT are also commercially available in many countries. The development of manufacturing human-identical milk oligosaccharides along with the functional studies of HMOs are important priority areas in glycoscience.