The role of the brain in the co-ordination of vegetative functions like respiration and blood circulation and for higher functions like learning, memory, recognition, perception or planning of motor activity requires a highly complex system. The brain performs the tasks named above and many more, if it is continuously provided with a high amount of oxygen and glucose. As this organ has no energy reservoir, brain functions collapse within seconds when the energy supply is interrupted. The damage becomes irreversible when the supply of the brain stops for 8-10 minutes. In order to explain this susceptibility of the brain, the pres-ent text starts with a short survey on functional anatomy and the structure and function of nerve cells. In a further paragraph the energy demand of this organ is described. The last paragraph deals with the diagnosis of brain death. This irreversible loss of function of the brain is only a partial death if other organ functions survive in artificially ventilated patients treated with intensive care. The loss of brain functions, however, means the loss of all qualities characteristic of human beings. Therefore, there is widespread consensus that the death of the brain allows the conclusion that such a patient is dead. The patient’s relatives may doubt that his status is irreversible. These doubts should stimulate medical profession-als to explain with empathy what has happened to the patient.