The causal relationship between feed supply and fat and lean growth is critical to effective determination of optimum feed supply to growing pigs. Although excessive fatness may be controlled by reducing feed intake, in early growth appetite limitation is such as to ensure a linear lean tissue growth response to increasing feed supply. For pigs of high intrinsic merit (entire males and animals of superior genotype), feeding to appetite may be justified until high livemass. The more likely the animal is to fatten, the earlier should restriction be imposed. For young growing pigs it is apparent that appetite is unnecessarily depressed in the commercial production environment. Studies of mass stasis and negative growth in newly weaned pigs have demonstrated a remarkable ability for fatty tissue catabolism. Experiments examining the responses of breeding sows to different levels of feeding in pregnancy and lactation point to the importance of maintaining sow body condition, and the unavoidability of some level of fat losses during lactation. The prediction of animal response to nutrient supply requires effective determination of the energy content of feed ingredients and compounded diets; the best prediction equations appear to favour the analysis of neutral detergent fibre rather than crude fibre.