Blood component therapy "Nach und mit Mass" = Preparation and Use of the Right Amount. Blood preservation, which is done using citrated blood to prevent coagulation, has been in use for more than 30 years in our hospital. Without stored blood modern surgery would be impossible. The use of plastic bags has made it possible to separate blood into its components. This is done in a closed system. Using blood components--blood cells and plasma derivatives--leads to the development of a more specific haemotherapy. This is done by first of all, isolating the blood cells and also separating plasma into its different components. The stored blood undergoes alterations as time passes by; this has to be considered according to indications such as for fresh blood ("warm blood") as well as for stored fresh blood, red blood cells (erythrocyte concentrates) or deep-frozen fresh plasma. In blood replacement, on other hand, for stopping of any bleeding you must consider the exact cause and indication. The use of micro-aggregate filters has to be considered. Generally, when there is an indication for blood transfusion, it is most important to consider also the "right amount". One should not only consider the indication for blood transfusion but at the same time possible errors. Each surgeon should periodically review the most important questions and problems of "transfusion medicine" such as the surgeons had to do who lived in previous decades.