This report reviews the energy and protein requirement systems for dairy cows currently used in Britain and presents a new approach, along with the outline of a model for its implementation. Current systems give little consideration to the responses of the animal to changes in the nutrient supply, or to interactions between dietary constituents in their effects on digestion and metabolism. Neither do they predict any effect on the partition of nutrient use between milk and body, or changes in the yields of milk fat, protein and lactose, which determine the commercial value of the milk. Therefore the systems, although not inaccurate, lack relevance to the current needs of UK milk producers. Models which embody concepts of nutrient supply and utilization are reviewed and their development as viable alternatives is considered. It is concluded that a new diet formulation system for dairy cows should aim to predict voluntary feed intake, the partition of nutrient use between milk production and tissue deposition, and the short and long-term of effects of nutrition on fat, protein and lactose yields. The physical and biological characteristics of the cow must also be recognized and incorporated into any model for response prediction. The report is also published in Nutrition Abstracts and Reviews - Series B: Livestock Feeds and Feeding. It is an authoritative review for advanced students, research workers and advisors in animal nutrition and in dairy science and technology.