Over the years, the β-glucan of oats and barley has been the subject of study either because of the importance of the cholesterol-lowering potential to health claims (FDA 1997, 2005) or, in the case of barley, because of the role of β-glucan and β-glucan-rich endosperm cell walls in malting and brewing. β-Glucan is also present in rye and in much lesser amounts in wheat. The most striking difference in these latter two sources is the difficulty in extractability; alkali rather than water is required for significant release from the cell walls. This review will discuss physicochemical properties of oat and rye β-glucan and, where information allows, relate these to physiological effects. Viscosity, or more generally rheology, plays a central role in discussions of cereal β-glucan functionality and physiological effects and will be the focus of this review.