“Mixed dementia” is traditionally defined as Alzheimer's disease with cerebrovascular disease (CVD). Because the risks of both neurodegenerative dementias and cerebrovascular disease increase with age, the mixed dementias are likely the most common. In practice, patients with mixed dementia are diagnosed by one of two routes: Either they have evidence of a neurodegenerative dementia and CVD at the outset, or, they have a classical neurodegenerative presentation but are found to have ischemic lesions by neuroimaging. These facts have implications for the development of evidence-based diagnostic criteria.