Freedom of will is the prerequisite for legal capacity. In adults, it can be repealed only by a psychiatric illness. On the part of the jurists a restrictive handling is stipulated. A theory of will and action accepted by jurisprudence and psychology/psychiatry in like manner is missing. From the viewpoint of the psychiatric expert, the diagnosis presents a frame of orienting lines, in which the analysis of the psychopathological findings yields the basis of judgement. The proceeding is described for the particular groups of diseases. Thereby the specific problems are discussed and proposals to solve them are drawn up. Further, the prerequisites for the capacity to give consent to methods of medical treatment are commented.