Since the early attempts of Whittemore and Fawcett (1974) a lot of progress has been made in our ability to predict the responses of pigs to nutrients. There are several simulation models that predict the performance of pigs under ideal environmental conditions when they are given access to a defined amount of food of a certain composition. Some of these models form the basis of a number of decision support systems used by the industry. A smaller number of models are able to predict the food intake of pigs, as well as their performance on foods of different compositions. These models agree that the pig needs to be described by two dimensions, at least, that relate to its maximum capacity to deposit lean tissue and its genetically determined level of fatness. The issue of whether pigs differ genetically in the manner that they partition limiting nutrients is a more contentious one.