Over 800 000 people in the UK are demented. Alzheimer's disease, dementia with Lewy bodies, vascular dementia and frontotemporal lobar degeneration account for the majority. Although detailed clinical assessment forms the basis of evaluating a patient with cognitive impairment, structural and functional imaging techniques are increasingly being used. Neuroimaging can identify changes to supplement the clinical diagnosis and help to distinguish dementia subtypes. This may be important for treatment, prognosis and care planning. Furthermore, early changes on structural and functional imaging may have a role in preclinical detection, perhaps allowing people to start any treatments early. In this review, we explain the tools available to the neuroradiologist and examine the implications of imaging findings in assessing patients with cognitive impairment or dementia.