The availability of xylan-type polysaccharides, representing an immense resource of biopolymers for practical application, is summarized. Xylans constitute 25-35% of the dry biomass of woody tissues of dicots and lignified tissues of monocots and occur up to 50% in some tissues of cereal grains. The most potential sources of xylans include many agricultural crops such as straw, sorghum, sugar cane, corn stalks and cobs, hulls and husks from starch production, as well as forest and pulping waste products from hardwoods, in particular. The structural diversity and complexity of xylans is illustrated and shown to depend on the botanic source. Various extraction procedures suitable for the isolation of xylans from different plant sources are described and compared. It is suggested that certain structural types of xylans like glucuronoxylan, arabinoglucuronoxylan, and arabinoxylan can be prepared from certain plant sources with similar chemical and physical properties. In contrast to structural analyses, the physicochemical properties, including solubility, molecular weight and molecular weight distribution, and rheological properties have been studied only for few xylan types. From the functional properties, the thermophysical and tensioactive properties are described. Finally, the physiological activities of xylans, which represent important dietary fibers as well as the immunological activities of some xylan types, are presented.