This overview examines blood, blood components, their indications and contra-indications, from an anaesthetist's viewpoint. The dangers of any blood transfusion, including infection transmission and immune suppression, as well as the risks of massive and rapid transfusions, are discussed. Autologous predonation, intraoperative haemodilution and salvage are described to help prevent some of the risks of homologous blood transfusion. Preoperatively an acceptable individualised haemoglobin concentration should be calculated for each patient and a history for potential bleeding problems taken. In most patients perioperative anaemia does not adversely influence patient morbidity and mortality. However, if blood is required, 4 ml.kg-1 body weight of packed red blood cells will raise the patient's haemoglobin concentration by 1 g.dl-1. The bleeding time as a test of platelet function does not predict perioperative blood loss. However, it remains a useful test in patients with a known bleeding problem or in operations where even small amounts of bleeding increase the surgical difficulty and patient morbidity. If bleeding is due to thrombocytopaenia it is usually slow enough to allow time to check platelet number and function before ordering and transfusing them. Fresh plasma is a much overused product which should mainly be used for coagulation factor replacement, in adequate volumes (4-8 packs in dilutional coagulopathy). The well-informed anaesthetist should be better able to use blood products which, while they may be life saving, are neither innocuous nor inexpensive.