Noticing an elderly relative is suddenly starting to forget things, is getting confused or displaying sudden changes of mood are usually the first signs they are developing dementia.
But what if the disease could be spotted - and treatment given - years before any symptoms begin?
That's the aim of researchers coming together in a new centre at University College London.
Dementia is the sixth leading cause of death in the UK, and a major cause of disability. Those affected often have to move into residential care.
About 750,000 people are currently affected, and that is set to rise to a million by 2021.
The research team will look at neurodegenerative diseases - including Alzheimer's disease, as well as Parkinson's disease and less common but equally devastating conditions such as Huntington's disease and Motor Neurone Disease.