Iron biomarkers were developed to define the size of iron stores and the adequacy of the iron supply required to meet functional needs. Approximately 80% of the iron delivered to tissues through the circulating plasma pool will be incorporated into hemoglobin. Consequently, with the exception of serum ferritin, iron biomarkers are measures of iron sufficiency for erythrocyte production. They have proven to be very valuable in the determination of the cause of anemia in the clinical setting in which additional information about factors that affect the patient's health is available. However, all current biomarkers are affected by factors other than iron status, which limit their utility for the determination of the prevalence of iron deficiency in some populations, particularly in populations who live in developing countries. Furthermore, relations between iron status and functional outcomes such as neonatal and infant mortality; motor, cognitive, and emotional development in infants; and severe morbidity from malaria in young children are inadequately characterized. There is a need to identify and standardize biomarkers that have the highest predictive value for specific functional outcomes in each setting. The most appropriate biomarkers may vary with the setting and be influenced by age, sex, gestational stage of pregnancy, and environmental factors such as repeated or chronic infections. There is also an urgent need for improved technology to permit the use of specific biomarkers in field studies in resource-poor regions. Finally, more research is required to define the potential role of hepcidin and non-transferrin-bound iron assays.