This chapter examines interactions of dietary fibre components of pig diets with GIT microbiota, emphasizing cereals and legumes fed to pigs. Carbohydrate composition of these feedstuffs are described, and their relationship to metabolic activity of the porcine intestinal microbiota and interactions with the host. Fermentable carbohydrates which act as substrates for microbial metabolism are described, followed by an assessment of cereals and legumes as potential modulators of intestinal microbiota. Past work focussed on purified extracts, but attention is now focussing on whole grains or their fractions such as brans, in terms of effects on microbial populations. Such studies are showing the positive consequences of mixtures of DF in the form of complex plant cellular structures, rather than single refined ingredients, to achieve beneficial health outcomes. Further work is also needed to define appropriate quantities and types of DF to achieve desired effects whilst minimising negative outcomes.