Reported values for concentrations of micronutrients in human milk form the basis of the majority of micronutrient intake recommendations for infants and the additional maternal requirements for lactation. The infant recommendations may also be extrapolated to provide estimates for young children. The purpose of this review is to evaluate the adequacy of the milk micronutrient concentration data used by the Institute of Medicine to set recommendations for the United States and Canada, by FAO/WHO, the United Kingdom, and the European Food Safety Authority. The concentrations accepted by each agency are presented for each micronutrient accompanied by the source of information and comments on the number, location, status, and stage of lactation of the sample population, where known. These summaries show the small number of participants from which samples were collected in most studies, the wide range of concentrations within studies, the lack of longitudinal data, and the variability in collection methods. These factors contribute to the variability in nutrient intake recommendations among committees, although this variability is reduced by some committees that accept milk-composition values proposed by others. Values are also summarized from milk collected in studies in which mothers or infants were known to be deficient on the basis of clinical symptoms, biomarkers of inadequacy, or both, to show the extent to which milk micronutrients can be reduced by poor maternal nutritional status. We conclude that a new, multicenter study is needed to establish reference values for milk constituents across lactation.