Becoming demented is something people dread most about growing old; the image of a confused and wandering elderly person is a powerful and distressing one. Alzheimer's disease is the commonest cause of dementia, affecting 500,000 people in Britain alone. It is a progressive and presently incurable brain disease, causing great suffering and imposing an enormous burden on health resources. As a result, a large research effort is under way to find out what causes the disease, since a cure is only likely to emerge once the disease is understood. This research is wide-ranging and diverse, as the complexity of Alzheimer's disease requires many lines of enquiry if the puzzle is to be solved. At one extreme, the research focus is upon large groups of elderly people who are being followed up to see what features predict and herald the onset of dementia. At the other, individual genes, molecules and brain cells are under detailed investigation. Some of the latter studies have given important clues as to the cause of Alzheimer's disease, and these are briefly mentioned here.