The increasing prevalence of dementia with age, combined with the rapid aging of the population, heralds an oncoming epidemic of dementia in industrialized countries. The current prevalence of dementia over 65 years of age is close to 7% (2/3 Alzheimer's disease and 1/3 vascular dementia). Vascular dementia refers to a severe cognitive decline due to brain lesions of vascular origin. It is in fact a heterogeneous syndrome with multiple etiopathogenic varieties such as subcortical small artery disease, isolated strategic stroke, multiple infarcts, chronic cerebral hypoperfusion, and amyloid angiopathy. Vascular dementia classically occurs or worsens abruptly and has a subcortical pattern of cognitive impairment, contrasting with the gradual cortical dementia of Alzheimer's disease. Recent data, including the results of neuro-imaging studies, show however that this classical opposition is no longer justified and that most dementias are "mixed", with a synergy between Alzheimer's disease lesions and vascular risk factors going from, silent brain lesions of vascular origin, to at most, clinically patent strokes. The main therapeutic implication is that, in the absence of effective methods for Alzheimer's disease prevention, the best way to prevent dementia in general is to prevent strokes, based primarily on early treatment of vascular risk factors, particularly hypertension.