Melancholia is by far the commonest form of mental disorder that can be regarded as a true insanity, and from its general manageability is very often treated at home. A knowledge of it is therefore useful to the practising physician. There is another reason which increases the importance of this knowledge. Melancholia is always dangerous owing to the possibility of the patient taking his own life, and the presence of a tendency or desire to commit suicide is not sufficiently recognised by the members of the medical profession. It exists at one stage or another during the course of practically every case.