Stroke is a risk factor for dementia and dementia predisposes to stroke. Dementia prevalence in subjects with stroke is comparable with that seen in stroke-free subjects who are 10 years older. Although overall there is heterogeneity between studies of prestroke and poststroke dementia, stratifying by study method and clinical criteria (e.g. inclusion/exclusion of prestroke dementia, first ever vs. any vs. recurrent stroke) results in reasonably consistent estimates. Pooled dementia rates from studies of consecutive patients hospitalized with stroke indicate that around 10% have dementia prior to first stroke and 10% have new dementia in the first year after first-ever stroke with highest rates (over 30%) seen after recurrent stroke. After the immediate high-risk poststroke period, rates of new incident dementia are lower but remain elevated at around four times the background risk. Factors associated with prestroke and poststroke dementia are broadly similar but age, medial temporal lobe atrophy, female sex, and family history are more strongly associated with prestroke dementia suggesting a greater role for degenerative pathology. Poststroke dementia is associated with factors indicating a reduced cognitive reserve (prestroke cognitive decline, premorbid disability, low education, white matter disease, and atrophy) and is also strongly associated with stroke factors (lesion size, multiple lesions, and stroke recurrence) and complications of stroke (delirium, seizures, hypotension, systemic illness and incontinence) indicating the likely impact of optimal acute stroke care and secondary prevention in reducing the burden of dementia. Future studies are needed to clarify the interaction between degenerative, vascular, and systemic processes in the development of stroke-associated dementia.