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. Multiple neuropathologic processes may underlie dementia, including both neurodegenerative diseases and vascular disease. Under the current Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, dementia is considered a major neurocognitive disorder, in which a deficit in cognitive functioning is acquired rather than developmental. All dementia share common molecular mechanisms responsible for disease etiology and progression, such as hypoxia and oxidative stress, neuroinflammation, mitochondrial bioenergetics, neurodegeneration, and blood–brain barrier permeability. Dementia is most common in elderly individuals, with advancing age being the strongest risk factor. Furthermore, comorbidity is the rule rather than the exception for dementia in elderly persons