Glycan metabolism significantly influences the composition and function of the human gut microbiota. The gut microbiota, established shortly after birth, plays a crucial role in health by modulating immune development, digesting complex nutrients, and inhibiting pathogen colonization. Abnormalities in microbiota composition (dysbiosis) are linked to diseases such as inflammatory bowel disease, colon cancer, and obesity. Glycans, including those from diet, host mucus, and animal-derived tissues, provide a dynamic and heterogeneous environment for microbial growth. Microorganisms degrade these glycans into short-chain fatty acids (SCFAs), which serve as nutrients for gut cells and influence health. Dietary glycans, especially those from plant and animal sources, are critical in shaping the microbiota. The human gut microbiota is initially seeded by microorganisms from the birth canal and environment. In infants, the gut microbiota is dominated by Bacteroidetes, Proteobacteria, Firmicutes, and Actinobacteria, with HMOs in breast milk playing a key role in selecting specific bacteria. Formula-fed infants show different microbial profiles due to the lack of complex oligosaccharides in cow's milk. Post-weaning, the microbiota shifts to include more Bacteroidetes and Firmicutes, with dietary changes influencing microbial composition. Glycan availability and microbial adaptation to different gut regions (lumen vs. mucosa) determine microbial diversity and function. Microorganisms with broad glycan-degrading abilities (generalists) can shift metabolism based on dietary changes, while specialists focus on specific glycans. The gut microbiota is influenced by both dietary glycans and endogenous host glycans, which provide stable nutrients. The ability to degrade mucin glycans allows some bacteria to reside close to host cells, influencing colonic health. Microbial strategies for glycan acquisition include the Sus-like systems in Bacteroidetes and ABC transporters in Firmicutes and Actinobacteria. These systems enable bacteria to degrade complex glycans, including those in plant cell walls and host mucus. The gut microbiota also forms food chains, where bacteria interact to break down glycans and produce SCFAs, which have anti-inflammatory and antitumorigenic effects. The presence of certain microbial species can influence the gut microbiota's composition and function, with implications for health and disease. Understanding the role of glycans in shaping the gut microbiota is essential for developing strategies to improve health through targeted dietary interventions and microbial modulation. The diversity of glycans and microbial adaptations highlight the complexity of gut microbiota interactions and their impact on human health.Glycan metabolism significantly influences the composition and function of the human gut microbiota. The gut microbiota, established shortly after birth, plays a crucial role in health by modulating immune development, digesting complex nutrients, and inhibiting pathogen colonization. Abnormalities in microbiota composition (dysbiosis) are linked to diseases such as inflammatory bowel disease, colon cancer, and obesity. Glycans, including those from diet, host mucus, and animal-derived tissues, provide a dynamic and heterogeneous environment for microbial growth. Microorganisms degrade these glycans into short-chain fatty acids (SCFAs), which serve as nutrients for gut cells and influence health. Dietary glycans, especially those from plant and animal sources, are critical in shaping the microbiota. The human gut microbiota is initially seeded by microorganisms from the birth canal and environment. In infants, the gut microbiota is dominated by Bacteroidetes, Proteobacteria, Firmicutes, and Actinobacteria, with HMOs in breast milk playing a key role in selecting specific bacteria. Formula-fed infants show different microbial profiles due to the lack of complex oligosaccharides in cow's milk. Post-weaning, the microbiota shifts to include more Bacteroidetes and Firmicutes, with dietary changes influencing microbial composition. Glycan availability and microbial adaptation to different gut regions (lumen vs. mucosa) determine microbial diversity and function. Microorganisms with broad glycan-degrading abilities (generalists) can shift metabolism based on dietary changes, while specialists focus on specific glycans. The gut microbiota is influenced by both dietary glycans and endogenous host glycans, which provide stable nutrients. The ability to degrade mucin glycans allows some bacteria to reside close to host cells, influencing colonic health. Microbial strategies for glycan acquisition include the Sus-like systems in Bacteroidetes and ABC transporters in Firmicutes and Actinobacteria. These systems enable bacteria to degrade complex glycans, including those in plant cell walls and host mucus. The gut microbiota also forms food chains, where bacteria interact to break down glycans and produce SCFAs, which have anti-inflammatory and antitumorigenic effects. The presence of certain microbial species can influence the gut microbiota's composition and function, with implications for health and disease. Understanding the role of glycans in shaping the gut microbiota is essential for developing strategies to improve health through targeted dietary interventions and microbial modulation. The diversity of glycans and microbial adaptations highlight the complexity of gut microbiota interactions and their impact on human health.