It is increasingly recognized that host-derived glycans, such as milk oligosaccharides and mucin O-glycans, play pivotal roles in shaping the gut microbiota, being at least as important as dietary fiber (Fig. 70.1). Microbial enzymes involved in the degradation of mucin O-glycans and milk oligosaccharides have been identified and enzymatically characterized, and databases cataloging these enzymes and their roles in metabolic pathways have been developed and are continuously being updated. Certain human milk oligosaccharides have been commercialized to fortify infant formula in regions including Europe, the United States, and China. The mucus layer forms an interface between gut microbes and the host, and mucin degradation by gut microbes contributes to the establishment of both a healthy (eubiosis) and a disturbed (dysbiosis) gut ecosystem [1]. Recently, the immunomodulatory effects of aromatic amino acid metabolites and extracellular polysaccharides produced by gut microbes have attracted growing attention.