The bases of yeast classification are described, and the use of studies of the structure of macromolecules such as nucleic acids, proteins and polysaccharides in yeast taxonomy are discussed. Serological and chemical methods may be used in differentiating between different yeast species. A recent development which shows promise for studies in yeast taxonomy is the use of nuclear magnetic resonance spectroscopy of yeast mannans. The mannans are readily extractable, and the spectra, which give a characteristic "fingerprint" according to the structure of the mannan, are apparently distinctive characteristics of small groups of yeast species. The importance of yeasts and a deeper knowledge of their characteristics to the brewing and food industries is discussed briefly.