The standard for the assessment of the medical expert opinion is defined by the high demands expected from the judgement of a high court: Objectiveness, solid knowledge, self-criticism, in contestability in diction and firmness in the argumentation. From the legal point of view, the knowledge of the medical expert witness has to clearly go beyond the knowledge of his profession. The obligation for objectiveness is the basis of expert witness' activity. From the medical point of view, the physician has to take into account during the preparation of his expert assessment that structural deficiency of the personnel and surgical equipment of a department for surgery frequently plays a role due to the development of surgery to high tech medicine and the hospital physician cannot affect this. It is necessary for a physician as an expert witness to have basic knowledge about the evidence law and the medical liability process. On the other hand, judges and lawyers should basically know the medical way of thinking.