Iron deficiency is the most common global nutritional problem. Among the earliest functions to be affected are those associated with the brain enzymes involved in cognition and behavior and a number of immune mechanisms important for protection against infections. The adverse effects of iron deficiency during infancy on cognition appear to be irreversible. At all ages iron deficiency is intellectually and educationally disadvantageous, independently of ethnicity and of physical and social environment. Where iron deficiency is a public health problem dietary improvement and cereal fortification are recommended, and weekly iron supplementation has been demonstrated to be effective for preventing iron deficiency in vulnerable groups.