Iron deficiency remains a major health risk in the United States, despite the apparent availability of a high-quality diet. In the United States, at least 7.8 million adolescent girls and premenopausal women are iron-deficient. Worldwide, the challenge of identifying and treating iron deficiency is enormous. Physicians involved in the primary care and in the obstetric and gynecologic care of women of all ages must be aware of the nature of the problem and the correct approach to screening, diagnosis, and treatment. The potential benefit to newborns and infants and to their mothers is substantial. Furthermore, a thorough diagnostic evaluation has considerable potential for uncovering a potentially lethal disease, such as gastrointestinal malignancy, in a curable phase.