The main causes of iron deficiency (ID) are briefly discussed, followed by the examination of studies of ID and child cognitive and motor development and behaviour for evidence of a causal link, classifying them by study design. Iron deficiency anaemia (IDA) is associated with many psychosocial and economic disadvantages that can affect child development and could explain the frequently demonstrated relationship between IDA and poor development and behavioural differences. There is evidence of changes to brain function in infants with IDA. Many treatment trials lack statistical power due to small samples, including children without ID in the sampling or iron treatment resulting in little or no differences in iron status between placebo and treated groups. In children with IDA under 3 years, randomized trials indicate that iron supplementation is usually beneficial to motor development, but the effect on mental development is inconsistent. Iron supplementation also has a beneficial effect on cognitive function in school-aged children with IDA. Evidence for a threshold level of ID at which child development is affected is inconsistent, but children with IDA are most likely to benefit from iron supplementation. Possible harmful effects of iron supplementation in iron-replete children on growth and morbidity have to be considered when developing programmes and policy.