The paramount importance of proteins in nutrition and biochemistry is becoming increasingly recognized. A substantial portion of all enzymes is protein; this protein moiety often needs an accessory food factor or sometimes a mineral as a co-enzyme. The well-known pioneering role of Sir Frederick Gowland Hopkins in the development of the knowledge of accessory food factors or vitamins, and of enzymology, justifies the inclusion of the subject of this contribution in the Symposium. Nutrition, often erroneously regarded as being synonymous with dietetics, is a vast and rapidly growing subject. In relating these two large fields, i.e. proteins and nutrition, there will be time only to summarize some observations made in the Medical Research Council’s Human Nutrition Research Unit on (i) the determination of the protein values of foods as eaten by man, in terms which can be directly and quantitatively related to the protein needs of individuals; (ii) the production experimentally in animals of syndromes resembling two main forms of protein-calorie deficiency encountered in human beings; (iii) a demonstration of congenital defects, particularly in the central nervous system, occurring in the offspring of mothers fed on protein-calorie-deficient diets; (iv) the probable importance of protein-calorie deficiency in the pathogenesis of disease.