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The endosperm is the largest tissue of the grain. It is the primary starch and protein storage site and the source for white flour. It contains protein, carbohydrates, iron and B‐vitamins, such as riboflavin, niacin and thiamine. However, it is nutritionally inferior to the whole grain. The endosperm comprises two different parts: the starchy endosperm and the aleurone layer. The aleurone is botanically part of the endosperm. However, in practice, it is most often part of the bran since it is removed together with the pericarp and testa during the milling process. The main part of the grain constitutes the starchy endosperm, which consists of cells packed with nutrients, mainly starch, to support the growth of the embryonic axis after germination. It is also rich in protein. In all cereals, there is an inverse gradient involving these two components. The protein content increases towards the periphery, while starch is more predominant towards the center of the kernel. Cell size also decreases towards the outside and this is accompanied by increasing cell wall thickness. The endosperm cell walls of wheat are mainly composed of arabinoxylans, while in barley and oats (1‐3)‐ and (1‐4)‐β‐D glucans predominate. The contribution of cellulose to the cereal endosperm cell wall is low except in the case of rice.