Low birthweight (LBW), defined as birthweight <2,500 g, is a major global public health problem and is associated with lifelong cognitive and behavioral problems. Very LBW (VLBW) infants (<1,500 g) are at high risk of multiple macro- and micronutrient deficiencies, but most LBW infants are larger (1,500–2,500 g), and the most common nutritional problem of those infants is iron deficiency (ID). Globally, about 25% of pre-school children have ID anemia (IDA), the most severe form of ID, and there is good evidence that ID is associated with impaired brain development. However, adverse effects of excessive iron supplementation have been observed. Delayed umbilical cord clamping, which increases infant iron stores, should be recommended for all newborns. There is good evidence that intakes of 2 mg of dietary iron per kg daily prevents IDA in LBW infants without causing adverse effects. A recent study shows that this dose of iron supplementation also reduces the risk of behavioral problems at 3 years in infants with birthweights 2,000–2,500 g. VLBW infants need 2–3 mg/kg per day. To achieve these intakes, breastfed LBW infants should receive iron supplements, and formula-fed LBW infants should receive an iron-fortified infant formula.