The conception of deficiency diseases, particularly of the nutritional group is quite new. The discovery of the vitamins has opened a wide field in medical research. Gradual realization that many human diseases were not produced by infections or toxins but by vitamin deficiencies led to the conception of various deficiency diseases. Until recently the vitamins were only of clinical interest in the treatment of certain types of deficiency diseases but it is now realized that in large doses vitamins can be used in other conditions therapeutically. The next stage in the history of vitamin research consisted of distinguishing individual vitamins, their isolation, analysis and synthesis. Vitamins A and D apparently exist only in the bodies of vertebrates. Vitamin A can, however, be formed in the manmalian body from certain red and yellow pigments, carotenoids, which are to be found in many plants, micro organisms and vertebrates. Such substances are termed "provitamins".