Anemia may be the most common illness of critically ill patients. The majority of critically ill patients are anemic at admission to the intensive care unit (ICU), and hemoglobin concentrations typically decline during the first 3 days of ICU stay. Hemoglobin continues to decline for patients with sepsis and higher severity of illness. This patient population may be at particular risk of adverse consequences of anemia given the cardiovascular, respiratory, and metabolic compromise frequently encountered during critical illness. The etiology of anemia of critical illness is multifactorial, resulting from phlebotomy, gastrointestinal bleeding, coagulation disorders, blood loss from vascular procedures, renal failure, nutritional deficiencies,bone marrow suppression, and impaired erythropoietin response.