The major polysaccharides from plants include starch, mannans, and xylans. These have multiple levels of structure: with starch, for example, which is a highly branched glucose polymer, one goes from the individual chains (branches), to the whole branched molecule, to crystalline and amorphous structural features, growth rings, granules. and then the whole grain. Mannans and xylans have a chemical composition as an additional structural feature. Methods of characterizing these structures are discussed and the effects of various structural features on functional properties are examined. Chemical and enzymatic modifications change these structures, and thus can be used to give improved functional properties. The characterization of the resulting structural features is essential in guiding property optimization