Dementia is a consequence of brain disease. This article, the second in this series on dementia, discusses normal brain function and how certain functions are localised to different areas of the brain. This is important in determining the symptoms of dementia, depending on which parts of the brain are most directly involved. The most common types of dementia – Alzheimer’s disease, vascular dementia, dementia with Lewy bodies and frontotemporal dementia – affect the brain in different ways and cause different changes at the microscopic level. Dementia is affected by genetics, and recent advances in molecular techniques have improved our understanding of some of the mechanisms involved, which in turns suggests possibilities for new treatments in the future.