It is difficult in the majority of cases to estimate the correct intra-operative mass of fluid therapy for each patient und needs a high degree of clinical experience. Among circumstances and problems of surgery (kind of surgery, length of time, bleeding, loss of liquid via wound) and kind of anesthesia, are also the concomitant diseases of patients relevant for the action of all anesthetists. These concomitant diseases correct to detect and to deduce corresponding consequences calls likewise for high clinical experience. Furthermore, the pre-operative fasting is an important fact.