Before 2001, the World Health Organization (WHO) recommended that infants be exclusively breastfed for 4-6 mo with the introduction of complementary foods (any fluid or food other than breast milk) thereafter. In 2001, after a systematic review and expert consultation, this advice was changed, and exclusive breastfeeding is now recommended for the first 6 mo of life. The systematic review commissioned by the WHO compared infant and maternal outcomes for exclusive breastfeeding for 3-4 mo versus 6 mo. That review concluded that infants exclusively breastfed for 6 mo experienced less morbidity from gastrointestinal infection and showed no deficits in growth but that large randomized trials are required to rule out small adverse effects on growth and the development of iron deficiency in susceptible infants. Others have raised concerns that the evidence is insufficient to confidently recommend exclusive breastfeeding for 6 mo for infants in developed countries, that breast milk may not meet the full energy requirements of the average infant at 6 mo of age, and that estimates of the proportion of exclusively breastfed infants at risk of specific nutritional deficiencies are not available. Additionally, virtually no data are available to form evidence-based recommendations for the introduction of solids in formula-fed infants. Given increasing evidence that early nutrition and growth have effects on both short- and longer-term health, it is vital that this issue be investigated in high-quality randomized studies. Meanwhile, the consequences of the WHO recommendation should be monitored in different settings to assess compliance and record and act on adverse events. The policy should then be reviewed in the context of new data to formulate evidence-based recommendations.