Many transfusion reactions are iatrogenic in origin and reflect a lack of understanding of the indications for transfusion as well as a failure to appreciate what can be achieved with this form of therapy. The importance of anticipating problems before they arise is illustrated by a consideration of the indications for blood transfusion in several clinical situations: e.g. neonatal transfusion-exchange or replacement, cardiopulmonary bypass , specific organ failure (cardiovascular system, the liver and the kidney) and auto-immune haemolytic anaemia. Various potentially adverse reactions are considered primarily with a view to their avoidance.