There has been much interest shown among a group of chemists in the problem of determining the nature of the dietary essential, fat-soluble A, but little has been achieved in this direction. The history of the studies relating to this phase of nutrition is sufficiently interesting and instructive to warrant its inclusion as a special chapter. Biological tests have revealed the distribution of this dietary factor among our more important foods, and the relative abundance with which it occurs in a long list of articles of animal and vegetable origin is well known. Among the fats, the best sources are cod liver oil, butter fat and egg yolk fats. Some leaves of plants are known to be excellent sources, and there is good reason to believe that most of the unspecialized leaves (those not modified as storage tissues) contain it in relative abundance. In various parts of the world, grazing and browsing animals live exclusively or nearly so on the leaves of grasses or of trees, thus deriving their sole supply of fat-soluble A from this source.