Human milk oligosaccharides (HMOs) are the third most abundant solid nutrient in breast milk, following only lactose and lipids. HMOs form a variety of complex structures through different combinations of five basic components—glucose, galactose, N-acetylglucosamine, L-fucose, and N-acetylneuraminic acid—and possess multiple functions, including regulating the gut microbiota, establishing the intestinal barrier, boosting immunity, and enhancing cognitive development.