As important as defining nutrient requirements may be, supplying those nutrients using available ingredients must occur with the same precision. We are just beginning to understand the impact of variability, especially in energy content, on the accuracy of diet formulation. Finally, perhaps the most important development of the 1990’s has been the recognition that the economy is as important as the productivity; feeding programs are increasing evaluated as much on the cost per kg of gain, or cost per pig sold, or profit per pig space, as on days to market and average carcass index.