This is a lecture on fixed ideas and obsessions. For the genesis of these two lactors are necessary, first, an excessive condition of feeling; second, a congenital or acquired weakness of mental constitution. The difference between them and paranoia consists in the fact that in the case of obsessions there is no alteration of the personality. The author divides them into (1) obsessional emotions, (2) obsessional ideas, and (3) obsessional impulses. This is not a rigorous psychological distinction, but it is justified by the prevalence of one of the elements constituting each group. The emotional group may be divided into repulsions (or phobias) and imperative desires. The phobia may be general or for one determinate object only. Under the heading of obsessional desires are included dipsomania and certain other drug habits. The obsessional impulses are distinguished by their motor content; as these become more automatic they pass into the group of ties. Obsessional ideas have frequently a hypochondriacal basis. They are rarely accompanied by hallucinations. As regards prognosis there are two classes of cases, first, those in which the original psychical weakness is hereditary or dates from early infancy; second, the cases in which the weakness is acquired. In the first class the outlook is very grave and amelioration only results where development and education are very favourable. Other functions may through time become affected, but it rarely passes into other forms of insanity. Agitated melancholia may, however, develop. Suicide is rare, those affected being generally undecided. The treatment is largely general. Muscular and mental exercises are of benefit, as also outdoor work and electricity. If neurasthenia be present it is an indication for treatment; suggestion may also be tried.