Starch, a common constituent of higher plants, is the major form in which carbohydrates are stored. This chapter first introduces chemistry structure, synthesis, digestion, metabolism, and bioavailability of starch. Based on its digestion rate and extent, starch is classified into rapidly digestible starch, slowly digestible starch, and resistant starch. Resistant starch cannot be digested in the small intestine but can be fermented in the large intestine. This chapter introduces five kinds of resistant starch and commercially manufactured products and describes the fermentation process of resistant starch in detail, including the metabolism pathways, the bacteria involved, and end products. The fermentability of resistant starch depends on its physical and chemical structure. Particularly, short-chain fatty acids, mainly acetate, butyrate, and propionate, are produced during fermentation of resistant starch. These short-chain fatty acids have considerable bioactives. As a result, consumption of resistant starch has many benefits, including the prebiotic effect, decreasing protein fermentation, keeping colon healthy, the hypoglycemic effect, the anti-obesity effect, reducing inflammation and oxidative stress, improving mineral absorption, etc.