Arabinoxylans are a group of compounds with a basic structure consisting of a xylose backbone with arabinose side chains. Variations in structure occur as a result of variations in the xylose chain length, the ratio of arabinose to xylose and the introduction of alternative side-chains. This allows for an enormous potential range of structures. Arabinoxylans are major components of the cell walls of cereals. They have been reported to have numerous health benefits. This chapter presents a systematic description of the molecular features of arabinoxylans and relates these to the different extraction technologies used to obtain them. The proposal, that their immune modulation activity is related to their molecular weight and structure, is presented. Results demonstrating the effects of various arabinoxylans in various in vitro immunological tests are discussed.