This review was originally prompted by some recently published work of the Danish geneticist Winge which, it was felt, should be brought specially to the notice of readers of this Journal on account of its great importance and far-reaching implications.* The writer has endeavoured to give a very brief indication of some basic principles of genetics, the science which treats of the nature and mechanism of inheritance in all living organisms, since it is not possible to follow modern work on the biology of yeast without such a background. This is followed by a short account of Winge's and later Lindegren's genetical studies on yeast. It is common experience that yeast is a rather variable organism, the usual tendency being to attribute changes in its behaviour to uncontrolled variations in the conditions of its environment. The importance of yeast genetics lies in its demonstration that hereditary factors may be responsible for many alterations in the behaviour of a yeast stock; further, it offers means of control over yeast characteristics.