Despite the important place occupied by delusional thinking in psychiatric theory, comparatively little experimental work has been undertaken on this particular form of thought disorder. This is perhaps surprising considering its relevance to reasoning and belief as a social process, the role of the delusion in differential diagnosis, and the number of deluded patients available for investigation. Most investigators have been concerned with the content of delusions rather than their force, for the type of delusion has an undoubted influence on behaviour—quite clearly the patient with grandiose delusions acts differently from the paranoid individual; nevertheless, the available evidence suggests that the type of delusion reflects the individual's social history rather than the nature or degree of his psychopathology. More relevant to the latter is the degree of belief expressed by the patient in his delusions, the transitory (and malleable) ideas of an early depressive illness often contrasting with the implacable beliefs found in a condition of chronic melancholia. Some longitudinal studies have been undertaken recently in which the degree of delusional belief has been examined in terms of its resistance to manipulation, and this avenue of approach offers an extremely valuable scale of assessment of the individual psychiatric patient.