Beta-glucan is a dietary fibre found in some cereal grains, with particularly high levels in oats and barley. It has been shown to have numerous health benefits, such as a reduction in cholesterol and glycaemic response, the modulation of gut microbiota, the management of blood pressure and a reduction in the incidence of metabolic syndrome. The scientific evidence for these positive physiological effects is continually growing and the relationship between the consumption of barley and oat beta-glucan and blood cholesterol/lipid lowering is well established. Although numerous mechanisms have been proposed for the health effects of beta-glucan and are still under study, the primary factor thought to be responsible is the capacity of the soluble fraction of beta-glucan to form viscous solutions in the gastrointestinal tract. The viscosity of beta-glucan is linked to its molecular weight or size and its concentration in solution (which is influenced by dose and solubility). The physicochemical properties (i.e. molecular weight and solubility) and health effects of beta-glucan from oat and barley are influenced by the cultivar, the growing environment, processing, storage and the food matrix. It is therefore important for food researchers and industry to consider these factors when designing clinical trials and/or developing functional food products containing beta-glucan.