Infants who experience iron deficiency during the first 6-12 mo of life are likely to experience persistent effects of the deficiency that alter functioning in adulthood. A lack of sufficient iron intake may significantly delay the development of the central nervous system as a result of alterations in morphology, neurochemistry, and bioenergetics. Depending on the stage of development at the time of iron deficiency, there may be an opportunity to reverse adverse effects, but the success of repletion efforts appear to be time dependent. Publications in the past several years describe the emerging picture of the consequences of iron deficiency in both human and animal studies. The mechanisms for iron accumulation in the brain and perhaps redistribution are being understood. The data in human infants are consistent with altered myelination of white matter, changes in monoamine metabolism in striatum, and functioning of the hippocampus. Rodent studies also show effects of iron deficiency during gestation and lactation that persist into adulthood despite restoration of iron status at weaning. These studies indicate that gestation and early lactation are likely critical periods when iron deficiency will result in long-lasting damage.