The cereal grain is a complex organ consisting of several different tissues with specific physiological functions. Cell wall polysaccharides show a large diversity in composition, structure and organization within the grain tissues, but also among cereal species. Depending on their tissue of origin, these polymers exhibit very different functionalities (e.g. solubility, viscosity and gelling properties), which largely explains their impact in food processing and in human and animal nutrition. This chapter presents our current understanding of the structure and diversity of cell wall polymers occurring in the different tissues of mature cereal grains, along with their main physicochemical/functional properties. A major focus is on arabinoxylans and mixed-linked beta-glucans, which are the major polymers in the cell walls of the starchy endosperm of wheat, rye and barley. The structure and properties of heteroxylans in the outer tissues of the grain are described and the general structural features of cellulose and lignin are provided, with an emphasis on their possible interactions with heteroxylans.