Oligosaccharides represent a significant fraction of breast milk, reaching up to 20 g/l in early milk. Human milk oligosaccharides comprise close to 200 structures, which are not absorbed by the intestinal tissue and have no nutritional value for the breastfed infant. Early studies conducted around 1930 already attributed a prebiotic activity to milk oligosaccharides by showing their stimulatory effects on the growth of specific intestinal microbiota. In addition, milk oligosaccharides contribute to the defence against enteric pathogens by acting as soluble decoys preventing the adhesion of viruses and bacteria to their carbohydrate mucosal receptors. The structural complexity of milk oligosaccharides hampers the assignment of specific functions to single carbohydrates. The application of mouse models allows the investigation of unique milk oligosaccharides in the context of intestinal microbiota and mucosal immunity. In this respect, our recent work has demonstrated that uptake of the milk oligosaccharide 3-sialyllactose increases the inflammatory response observed in different colitis models. The proinflammatory action of 3-sialyllactose was attributed on the one hand to the modulation of intestinal bacterial groups, and on the other hand to a direct stimulatory effect on CD11c+ dendritic cells. The availability of pure oligosaccharides in large amounts will soon enable the study of these compounds in humans in the context of intestinal and metabolic disorders associated to various forms of dysbiosis.