Primary forms of dementia are those caused by neurodegenerative diseases, such as Alzheimer’s disease, Lewy body dementias, Parkinson’s disease dementia, frontotemporal dementia and vascular dementia. Rarer diseases causing dementia are progressive supranuclear palsy, corticobasal degeneration, Huntington’s disease, and Creutzfeldt-Jakob disease. Secondary forms of dementia include symptoms of dementia which are not caused by brain changes, but which occur as symptoms of other diseases or deficiencies. Thus, endocrinopathies, metabolic encephalitis, drug addiction and intoxication (Wernicke-Korsakoff syndrome), electrolyte imbalances, chronic infectious diseases, tumours, psychological disorders such as depression, and vitamin deficiencies can cause symptoms of dementia. The difference between primary and secondary dementia is relevant for treatment and therapy as the symptoms of secondary dementia are often reversible.