Iron deficiency (ID), defined as an insufficient supply of iron to the cells of the body after iron reserves have been exhausted, is the most prevalent single nutritional deficiency, affecting over 2,000 million people, mostly in the developing world. Infants, small children, adolescents and pregnant and fertile-age women are most vulnerable. Only about 50% of people with ID develop iron deficiency anemia (IDA), since this is a late manifestation of chronic ID. Based on the average daily iron requirement and on the rate of iron reutilization from red cell hemoglobin destruction, it can be estimated that after iron stores have been depleted, it takes about 4 months of ID erythropoiesis for adult women to have a drop in hemoglobin concentration [Hb] of 10 g/l, if the iron intake is only 70% of requirement. IDA can be defined by a [Hb] below an appropriate cut-off point for age, sex, physiological condition and altitude above sea level, or by a [Hb] increment of more than 10 g/l to the administration of adequate doses of iron. More than 85% of the nutritional anemias are IDA alone, or of iron combined with folate or other nutrient deficiencies.