The ultimate measure of the energy value of a pig feed is its net energy content, but this is not a function of the feed alone and depends on the pig and the use to which the energy is put. Metabolisable energy allows comparison with other species, but is also influenced by the growth characteristics of the animal and the level and source of protein in the diet. Digestible energy is considered more exact for the description of pig feeds, since it is largely free from animal effects. It is not sensible to determine in vivo the digestible energy content of all batches of compounded diets produced, but prediction might be possible from a knowledge of the nutrient composition of the ingredients and the ingredient composition of the feed. More practically, a laboratory method for the prediction of digestible energy content is required. Fibre has large effects on the digestibility of the diet and this fraction of the feed lacks sufficiently close definition. Further, the source and content of nitrogen-free extractive and lipids are significant elements in energy evaluation. Several equations have been produced to predict digestible energy content of feed from chemical composition and multiple regression equations have given best results, but those so far published tend to lose their accuracy when transferred to data from a wide range of feeds or to data from different centres. Nevertheless, despite it being impossible at present to choose a preferred equation to predict nutritive value, this approach is deserving of more detailed pursuit as being the only likely practical way for routine determination of the energy value of a large number of mixed diets.