Alzheimer's disease (AD) is a brain disease and is the most common form of dementia. Clinical trials for the prevention of AD have been moving to enroll subjects at increasingly earlier time-points, and are now focusing upon individuals who are not only cognitively normal but also have biomarkers associated with an increased risk of developing AD. Detecting one specific biomarker on brain scans, i.e. amyloid-beta protein, is currently used to inform diagnoses in cognitively impaired individuals, and its use may expand to pre-clinical AD cases as preventive therapies are developed.