Iron deficiency is a major worldwide health problem. There is recent evidence that the anemia is only the last manifestation of the syndrome and that symptoms occur before the anemia is manifest. Advances in outlining the physiology of iron deficiency have been made, gaps remain in the current understanding. While oral iron supplement remains the mainstay, some indications for the intravenous administration have developed. This review will highlight the epidemiology, physiology, clinical presentation, and treatment options.