Dementia is a symptom of a variety of specific structural brain diseases as well as several system degenerations. Alzheimer’s disease presently is the commonest cause in the developed world, causing a cortical-subcortical degeneration of ascending cholinergic neurons and large pyramidal cells in the cerebral cortex. Clinically, the disease reflects predominantly deterioration of function in the association cortex. Pharmacologically and pathologically, abnormalities are more diffuse and extend into sensorimotor cortical areas as well.