Present paper explains structure, aims and physiological bases of the Cornell Net Carbohydrate and Protein System. The system was thought to formulate rations for dairy and beef cattle. From 2000 the system was adapted to formulate for dairy ewes and growing lambs, but its reliability is not been sufficiently verified yet. The system is articulated in four sub-models containing equations that estimate microbial protein synthesis and passage of feed carbohydrate and protein fractions. This information is successfully used as a basis for predicting ME and protein MP of rations. This review examine the sub-models relatives to feed fractions, rumen and gut digestion and estimation of TDN of the diets.