The relationship between four micronutrient deficiencies (iodine, iron, zinc and vitamin B-12) and children's cognitive functioning is reviewed. Iodine deficiency during pregnancy has negative and irreversible effects on the developing fetus. Although there is some evidence that postnatal iodine deficiency is associated with cognitive deficits, the findings are controversial. Iron deficiency is widespread and has been associated to cognitive deficits, but the results of prevention trials are inconsistent. Zinc deficiency has been linked with low activity and depressed motor development among the most vulnerable children. Associations with cognitive development are less clear and may be limited to specific neuropsychological processes. Vitamin B-12 deficiency has been associated with cognitive problems among the elderly, but little is known about its effect on children's cognitive functioning. Rates of vitamin B-12 deficiency are likely to be high because animal products are the only source of vitamin B-12. Although micronutrient deficiencies often co-occur in the context of poverty, little is known about the impact of multiple micronutrient deficiencies on cognitive development.