Human milk oligosaccharides (HMOs) are a diverse group of structurally complex, unconjugated glycans unique to human milk. Initially discovered as a prebiotic "bifidus factor" that promotes the growth of beneficial bacteria and shapes the intestinal microbiota, HMOs have been found to have additional beneficial effects on the breastfed infant. These include acting as antiadhesive antimicrobials, preventing pathogen attachment to mucosal surfaces, reducing the risk of viral, bacterial, and protozoan infections, modulating epithelial and immune cell responses, and potentially providing essential nutrients for brain development and cognition. HMOs also exhibit protective effects against necrotizing enterocolitis (NEC) in preterm infants. Most of the current knowledge about HMOs comes from in vitro, ex vivo, and animal studies, with limited data from human intervention studies. The review highlights the historical origins of HMO research, the structural diversity and biosynthesis of HMOs, and their potential benefits to the breastfed infant. It also discusses the differences in HMO composition between human milk and milk from other mammals, as well as the challenges and future opportunities in HMO research.Human milk oligosaccharides (HMOs) are a diverse group of structurally complex, unconjugated glycans unique to human milk. Initially discovered as a prebiotic "bifidus factor" that promotes the growth of beneficial bacteria and shapes the intestinal microbiota, HMOs have been found to have additional beneficial effects on the breastfed infant. These include acting as antiadhesive antimicrobials, preventing pathogen attachment to mucosal surfaces, reducing the risk of viral, bacterial, and protozoan infections, modulating epithelial and immune cell responses, and potentially providing essential nutrients for brain development and cognition. HMOs also exhibit protective effects against necrotizing enterocolitis (NEC) in preterm infants. Most of the current knowledge about HMOs comes from in vitro, ex vivo, and animal studies, with limited data from human intervention studies. The review highlights the historical origins of HMO research, the structural diversity and biosynthesis of HMOs, and their potential benefits to the breastfed infant. It also discusses the differences in HMO composition between human milk and milk from other mammals, as well as the challenges and future opportunities in HMO research.