At the meeting of the American Society of Biological Chemists in December, 1919, the matter of a classification of the fats and related substances was brought before the members for discussion. It was decided that the available knowledge on the subject was insufficient to justify a classification at that time. While admitting the truth of the conclusion it has seemed to me since, that something might be gained by an attempt at classification,—in clarifying our ideas and in bringing the newer developments in the field into connection with the old—even though the scheme might later have to be radically changed or even abandoned. It is becoming more and more apparent, for example, that the fats and the substances ordinarily grouped under the name of lipoids are so intimately related both chemically and in metabolism—all being directly connected with the metabolism of the fatty acids—that they should be considered together, and when so considered they form a group which is believed to be as distinct and well ...