β-Glucan, a naturally occurring polysaccharide, is found mostly in the cell wall of fungi. In macroorganisms it acts as a potent biological response modulator. Its history began in the sixties of the last century, when it was found as a main component of zymosan as well as of extracts from medicinal Asian mushrooms. The review itemizes sources, structure, mechanisms of biological effects and adverse reactions of ?-glucan and proposes alternatives of the future research.