Feeding infants breast milk of healthy mothers is associated with a lower incidence of infectious and allergic diseases. Although this effect is of multifactorial origin, it is widely accepted that the entire intestinal flora of breast-fed infants provides antiinfective properties and is an important stimulating factor for the postnatal development of the immune system. The effect of human milk on the postnatal development of the intestinal flora cannot be attributed to a single ingredient. It is generally accepted, however, that human milk oligosaccharides play a key role in this matter. Apart from their prebiotic effects, there is also evidence that human milk oligosaccharides act as receptor analogs to inhibit the adhesion of pathogens on the epithelial surface and interact directly with immune cells. Because of their complexity, oligosaccharides with structures identical to human milk oligosaccharides are not yet available as dietary ingredients. In the current search for alternatives, non-milk-derived oligosaccharides have gained much attention. As 1 example, a mixture of neutral galacto-oligosaccharides and long chain fructo-oligosaccharides have been identified as effective prebiotic ingredients during infancy. Furthermore, another class of oligosaccharides with a potential physiological benefit could be those found in animal milks. Most of the oligosaccharides detected in domestic animal milks have some structural features in common with human milk oligosaccharides. One important fact is the occurrence of sialic acids such as N-acetylneuraminic acids. However, total amounts and individual structures are still different from those in human milk oligosaccharides. Although these structural similarities between animal milk and human milk oligosaccharides are promising, further studies are needed to prove the equivalence of their function.