The senile psychoses are in regard to their origin an unsolved problem. Much valid work has been done in the field of anatomy and histology in approaching this problem, but as the mental diseases of old age cannot be separated from the growing old of body and mind generally, this problem is more or less a biological one. The borderline between old age and senile dementia is not a sharp one. In brains of old people without clinical symptoms of dementia there have been found histological changes, such as are usually found in cases of senile dementia. This makes it probable that the extent of degeneration of brain cells alone is not decisive for the appearance of senile psychoses. The finding of the characteristic plaques in the brain of senile psychoses may give evidence of the extent of the process and the severity of clinical symptoms, but it does not tell us anything about their nature and origin. Whether a constitutional factor is decisive or whether the histological changes are a reaction to an unknown noxa is undecided. Heredity may have its share too.