Maternal weight homeostasis during lactation depends on the management of energy resources. Studies indicate that regulatory processes allow for successful lactation in varied conditions of food availability. These processes involve mobilizing fat, increasing food intake, reducing energy expenditure, and changing the composition or volume of milk. Changes in energy efficiency do not seem important. Early in lactation, fat mobilization appears to be physiologic and gradual, even when food is readily available. Later in lactation, dietary intake may decline even though that allows continued loss of body fat when fat has already lowered. Where increasing dietary intake is less possible, or dietary restriction is imposed, reductions in energy expenditure seem to take precedence over an increase in the rate of fat mobilization. The findings reviewed indicate that for lactation to play a major role in the reduction of body fat in the postpartum period, women have to breastfeed fully for a substantial period.