Although the risk of developing dementia increases with age, onset can be as early as the third or fourth decade of life. Genetic influences play a more important role in younger than in older people with dementia, so young onset dementia may cluster in families. Diagnosing young onset dementia is challenging. The range of possible presenting features is broad, encompassing behavioural, cognitive, psychiatric and neurological domains, and symptoms are often subtle initially. Frequently the complaints are misattributed to stress or depression, and the patient is falsely reassured that they are too young to have dementia. The most common causes of young onset dementia are early onset forms of adult neurodegenerative conditions and alcohol. Vascular dementia is the second most common cause of young onset dementia after Alzheimer's disease. Conventional vascular risk factors may be absent and diagnosis relies on imaging evidence of cerebrovascular disease. Obtaining a detailed history remains the most important part of the workup and usually requires corroboration by a third party. Undertaking a basic neurological examination is also important. Those with suspected young onset dementia should be referred to a neurology-led cognitive disorders clinic where available as the differenti diagnosis is considerably broader tha in older adults and requires specialist investigation.