Like other mental disorders, mental deficiency presents great problems in diagnosis. These diagnostic problems derive ultimately from the fact that a medical diagnosis can be made only in exceptional cases. Mental retardation is not a disease in itself, but only a symptom which may be found in a number of different conditions. It is true that there are a great number of oligophrenia syndromes with known aetiology, but together they represent a numerically small group. When one considers, in addition, that no clear cut distinction between the normally intelligent and the mentally retarded can be made, since the difference is merely quantitative, the difficulties of diagnosis become manifest.