The energetic efficiency of the sow is closely related to her productivity. This review suggests that a system of feeding that attempts to maintain true weight as measured by total energy content of the body of sows during lactation and to allow no gain in weight above that accounted for by the fetuses and fetal membranes during gestation will allow the maximum energetic efficiency. Changes in water balance during lactation and gestation and possible changes in fat-muscle ratios during these periods should not be confused with energy maintenance. For gilts, and possibly for 2nd litter sows, an energy allowance for normal growth would also be required. To lower maintenance requirements, relatively low mature weights are desirable. Direct energy conversion by the young pig by allowing it to eat as soon as possible should also be encouraged as an aid to total energetic efficiency.