This review synthesizes the evidence on the effect of iron supplementation in infants and children on early and long-term child cognitive development. The findings indicate a potential benefit of oral iron treatment for cognitive performance in anemic primary-school children. However, antenatal and early childhood oral iron intervention studies show inconsistent effects on early and long-term childhood cognitive outcomes. The authors suggest that 1) effects from oral iron on cognitive development in young children are small or nonexistent or 2) heterogeneity between trials and the low quality of many studies make assessment of effect difficult, highlighting the lack of large, placebo-controlled trials in children under 2 years in low-income settings.