It has become increasingly apparent, especially with the advent of MRI brain scanning, that a large number of patients develop signal intensity changes in the subcortical white matter and periventricular region as they age. This appears to be accelerated by risk factors for small vessel cerebrovascular disease such as hypertension, smoking, diabetes mellitus and hyperlipidemia. The major question becomes when such changes become clinically significant. It is obvious that subcortical lacunar-type infarction can be identified by the clinical presentation. For example, typical examples of so-called "lacunar syndrome" include pure motor hemiparesis, pure sensory stroke, sensorimotor stroke, clumsy hand-dysarthria, and hemiataxia-hemiparesis. The issue becomes a measure of impact on functional ability. This is influenced by several factors. Baseline IQ and educational level, as well as expectations of age, certainly play a role. A person who develops cognitive impairment and long tract signs in their 50s or 60s is certainly going to be recognized as more impaired than an 80 year old individual who is retired and primarily is engaged in recreational activity. It would be expected that a person born with limited intellectual capacity and/or limited educational opportunity would be less likely to be identified as impaired than a person who has achieved substantial economic achievement through their innate talents. The concept of tissue loss or lesion load becomes important when determining how pronounced the ischemic cerebrovascular changes translate into functional impairment. Correlative pathology may include cortical atrophy and ventricular dilatation. Loss of either cortical or subcortical tissue function is expected to be related to functional compromise. In addition, there are potential features such as the coexistence of small vessel cerebrovascular disease and Alzheimer's disease. Small vessel cerebrovascular disease might also play a contributing factor in patients susceptible to Dementia with Lewy Bodies or patients susceptible to fronto-temporal dementia or any other dementing process. Thus, the concept of tissue loss or lesion burden of disease becomes increasingly important as we recognize the potential for multifactorial issues, including genetic factors, to contribute to the phenotypic expression. The relationships between cognitive impairment, dementia and subcortical vascular lesions are poorly understood. There have been several papers on the different aspects of cerebral insults and their impact on cognition, the various kinds of dementia and different methods of analyzing the impact of the various insults to the brain. This chapter is an attempt to review all pertinent information currently available on the poorly understood condition of "subcortical ischemic cerebrovascular dementia."