Diet-derived carbohydrates that are not fully digested in the upper gut, known as nondigestible carbohydrates, provide a major source of energy for bacteria that colonize the human large intestine. It is well established that dietary intake of nondigestible carbohydrates influences microbial fermentation and total bacterial numbers in the colon. Recent evidence from molecular ecology has also shown that the amount and type of nondigestible carbohydrates (e.g., resistant starch, non-starch polysaccharides, and prebiotics) influences the species composition of the intestinal microbiota both in short-term dietary interventions and in response to habitual long-term dietary intake. Interindividual variation in gut microbiota may, in part, reflect differences in dietary intake, but the response of the gut microbiota to dietary change can also differ among individuals. As a better understanding is gained of the impact of different groups of bacteria on host metabolism, the ability to manipulate the microbiota through diet should provide a route for delivering health benefits.