This chapter summarizes both human and animal data indicating that nutritional deficits can be devastating if they occur during certain "critical periods" prenatally or in the first 3 years of life. To meet the metabolic and nutritional demands of pregnancy and lactation, a woman whose weight is normal prior to pregnancy must gain approximately 25 pounds. Under normal conditions, the adult brain depends almost entirely on aerobic metabolism of glucose for its energy supply, and decreased supplies of either oxygen or glucose have negative effects on brain function. The earlier the malnutrition occurs, the greater the reduction in brain size. The mediating factors in catch-up brain growth in humans also remain uncertain, but judging from animal data, both nutritional and environmental enrichment are involved. It is reasonable to expect that nutritional deficits occurring early in development will also be time specific in their effects and depend on the phase of anatomical and biochemical maturity at the time of the deficit.