To date, RBC transfusions have been proven effective in patients with profound anaemia or circulatory shock. However, the impact of the storage process and the so-called storage lesion on oxygen-carrying properties and, hence, the efficacy of RBC transfusion regarding tissue oxygenation are much debated at present. Alterations of RBC physiology have been comprehensively described ex vivo. Reduced deformability, increased adhesiveness and aggregability of stored RBC impair their rheological properties; anaerobic cellular metabolism with reduced contents of 2,3 bisphosphoglycerate and adenosine triphosphate (ATP) increases oxygen affinity and impairs oxygen release to the tissues.