Cereal β-glucan describes a group of soluble dietary fiber primarily found in barley and oats. It is characterized by the mixed occurrence of β-(1,3) and β-(1,4) bonds between the glucose monomers, forming long polysaccharide chains with unique properties. Alongside their technological benefits, they exhibit a number of health-beneficial properties. Nevertheless, β-glucan applications as nutraceutical food ingredient, are rarely utilized. The changes in physicochemical (molar mass, solubility and viscosity) and health-beneficial (glycemic control, lowering of blood glucose) properties during food processing and storage are a major drawback. The understanding of the complex mechanisms behind the health benefits of β-glucan is still incomplete. In contrast to original believes, it becomes evident from recent literature, that physiological properties are not only based on the dose administered. A more thorough view on the molecule and its structure-size-function relationship is required to understand the impact of molar mass, molar ratio, solubility and viscosity. Therefore, this review evaluates the recent scientific data to identify β-glucan key properties responsible for the nutraceutical function in the final product. This provides a guideline for future research which characteristics need careful monitoring and preservation throughout processing and help to fully utilize cereal β-glucan as nutraceutical ingredient.