Approximately 14-40% of patients in industrialized countries present with preoperative anemia. Depending on the severity, anemia is associates with increased perioperative morbidity and mortality. One of the most important causes of preoperative anemia is iron deficiency which is usually easy to treat. Implemented in the multimodal concept of patient blood management, the diagnostics and treatment of preoperative anemia are important aspects for improvement of perioperative outcome. Adequate and early diagnostics of the cause of anemia before treatment is important because treatment options, e.g. with iron, erythropoetin, folic acid and vitamin B-12, may be expensive, may have severe side effects, and in the case of a wrong indication, will not improve anemia. In addition, an adequate regeneration of the erythrocyte volume requires time. This review article presents important aspects of the epidemiology and prognostic implications of preoperative anemia, the physiology and pathophysiology of anemia as well as diagnostic features and the evidence base for preoperative treatment options.