Oligosaccharides are abundant in human milk. Production of these highly diverse structures requires significant energy expenditure by the mother and yet these human milk oligosaccharides offer no direct nutritive value to her infant. A primary function of human milk oligosaccharides is to shape the infant's intestinal microbiota with life-long consequences. Bifidobacterium longum subspecies infantis (B. infantis) is unique among gut bacteria in its prodigious capacity to digest and consume any human milk oligosaccharide structure, the result of a large repertoire of bacterial genes encoding an array of glycosidases and oligosaccharide transporters not found in other bacterial species. In vitro, B. infantis grows better than other bacterial strains in the presence of human milk oligosaccharides, displays anti-inflammatory activity in premature intestinal cells, and decreases intestinal permeability. In premature infants, B. infantis given in combination with human milk increases B. infantis and decreases Enterobacteriaceae in the feces. Probiotics containing B. infantis decrease the risk of necrotizing enterocolitis in premature infants. Colonization with B. infantis is also associated with increased vaccine responses. Probiotic organisms have historically been selected based on ease of production and stability. The advantages of B. infantis, selected through coevolution with human milk glycans, present an opportunity for focused manipulation of the infant intestinal microbiota.