Nutrition remained largely an art until the scientists entered the field and chemists described feeds in terms of chemical entities. The physiologists, endocrinologists and bacteriologists joined the team and this paved the way for demonstrating that the feeds are heterogeneous combination of nutrients. It was shown that most single feed is unbalanced in terms of an animal's requirements. The field of nutrition has moved forward at an accelerated pace, year by year, as the new knowledge accumulated. In preparing this manuscript, the general framework and character of the book established by seniors, have been preserved. The text presents the principles of nutrition and their applications to feeding practice. It not only presents facts but shows how many of these have been obtained, illustrating experimental methods which will continue to develop new facts in the future. Like other rapidly advancing fields, new discoveries in the science of nutrition, which add to our knowledge, inevitably cause some modification in ideas previously held. The literature citations have been made on the basis of their usefulness to those students who are the principal users of the text. Such a compilation has resulted in an unbalanced representation of nutrition research on a worldwide basis.