Cow-milk based infant formulas are commonly supplemented with non-digestible carbohydrates (NDCs) to resemble the functional properties of human milk oligosaccharides. NDCs support the development of a balanced immune system through their direct interaction with immune cells, but also through their fermentation by gut microbiota. In this thesis, 7 structurally distinct NDCs were studied using an in vitro batch fermentation set-up with faecal inoculum of 2- and 8-week-old infants. The degradation of the NDCs, changes in microbiota composition and production of organic acids were studied, as well as the effects of the fermentation digesta on the cytokine production by dendritic cells. The large number of NDCs included in this study together with the focus on the fate of individual oligomers present in the NDCs increased our understanding of their structure-specific fermentability by infant faecal microbiota and concomitant immune effects in infants.