The advent of human amyloid imaging represents a research breakthrough in Alzheimer's disease (AD). It is now possible to detect the early stages of cerebral amyloidosis, a major pathologic component of AD, in living humans using positron emission tomography (PET). This technology will likely enable earlier AD diagnosis, but further research is required to determine whether a positive amyloid PET scan predicts imminent decline in questionably or mildly impaired individuals, and whether amyloid PET can be used to track the efficacy of emerging antiamyloid therapeutic agents. Initial human data are encouraging but suggest that individual amyloid PET findings should be interpreted cautiously, because cerebral amyloidosis precedes and does not equate with either clinical AD or pathologic criteria that define AD.