The development of intestinal microflora in newborns is strictly related to the kind of feeding. Breast-fed infants, unlike the bottle-fed ones, have an intestinal ecosystem characterized by a strong prevalence of bifidobacteria and lactobacilli. Data available so far in the literature show that, among the numerous substances present in human milk, oligosaccharides have a clear prebiotic effect. They are quantitatively one of the main components of human milk and are only partially digested in the small intestine, so they reach the colon, where they stimulate selectively the development of bifidogenic flora. Such results have been recently proved both by characterization of oligosaccharides in breast-fed infant feces and by the study of intestinal microflora using new techniques of molecular analysis, confirming that human milk oligosaccharides represent the first prebiotics in humans.