Cerebrovascular disease is currently viewed as a distinctly secondary cause of dementia that is of uncertain importance. Although it is commonly cited as the second leading cause of dementia, a lack of well-validated diagnostic criteria, which in turn reflects important gaps in fundamental knowledge about disease mechanisms, makes accurate epidemiology difficult. Two basic facts about vascular dementia share wide consensus. First, it is a disorder in which the pathologic features vary, spanning a wide spectrum in terms of type (from microscopic infarction to infarcts that derive from occlusion of major arteries) and location. Second, the pathologies of cerebrovascular disease and Alzheimer disease (AD) commonly co-occur, the latter being the usual primary cause of the cognitive and functional symptoms of these patients. Indeed, AD, as the predominant cause of dementia in North America and Europe, defines the backdrop against which research on vascular dementia proceeds. There is little interest in large cortical strokes as a cause of dementia, because the pathogenesis is apparent and the clinical issues are largely distinct from those of AD. It is dementia associated with small vessel disease in which the pathogenesis is not apparent and in which the differentiation from AD is difficult that has attracted most attention from researchers in dementia.