The following chapters are based on courses of lectures delivered at the London University and the Royal Institution during 1909–1910. In them an account is given of the work done on alcoholic fermentation since Buchner's epoch-making discovery of zymase, only in so far as it appears to throw light on the nature of that phenomenon. Many interesting subjects, therefore, have perforce been left untouched, among them the problem of the formation of zymase in the cell, and the vexed question of the relation of alcoholic fermentation to the metabolic processes of the higher plants and animals.

My thanks are due to the Council of the Royal Society, and to the Publishers of the "Journal of Physiology" for permission to make use of blocks (Figs. 2, 4 and 7) which have appeared in their publications.