This chapter discusses current hypothesis and conclusions regarding anemia and transfusion practice in critically ill patients. The first two sections introduce the pathogenesis of anemia in critically ill patients, which stems from inadequate bone marrow production and shortened red cell survival. Following sections describe oxygen delivery requirements and appropriate transfusion triggers. The last two paragraphs are comprised of a few selected topics of special interest such as age of stored red cells, transfusion triggers in pediatric critically ill patients and current transfusion practice in intensive care units.