The time course and order of the pathological-physiological processes in Alzheimer's disease (AD) are still under investigation and it is expected that molecular imaging will provide important insight into early brain pathology. Multi-tracer positron emission tomography studies visualizing fibrillar amyloid, inflammatory changes including astrocytosis and activation of microglia as well as cerebral glucose metabolism indicate that AD pathological processes are initiated and ongoing decades before the onset of cognitive symptoms. Therefore, prevention might be a new promising target for AD therapy.