The past two decades have not only witnessed great activity in the study of the various problems of animal nutrition, but they are especially distinguished by the new point of view from which these problems have come to be regarded. Speaking broadly, it may be said that to an increasing knowledge of the chemistry of nutrition has been added a clear and fairly definite general conception of the vital activities as transformations of energy and of the food as essentially the vehicle for supplying that energy to the organism. This conception of the function of nutrition has been a fruitful one, and in particular has tended to introduce greater simplicity and unity into thought and discussion. Much exceedingly valuable work has been done under its guidance, while it points the way toward even more important results in the future. The following pages are not a treatise upon stock-feeding, but are an attempt to present in systematic form to students of that subject a summary of our present knowledge of some of the fundamental principles of animal nutrition, particularly from the standpoint of energy relations, with special reference to their bearings upon the nutrition of farm animals. Should the attempt at systematization appear in some instances premature or ill-advised, the writer can only plead that even a temporary or tentative system, if clearly recognized as such, may be preferable to unorganized knowledge. The scaffolding has its uses, even though it form no part of the completed building.