Alzheimer's disease, the most common form of dementia in the elderly, is characterized by the progressive, global and irreversible deterioration of cognitive abilities. The development of positron emission tomography (PET) methodologies has made it possible to study the in vivo brain metabolic correlates of human cognitive and behavioral functions. Moreover, as PET scan examinations can be repeated, the progression of the neuropathological process and its relation to cognitive dysfunction can be followed over time. In an effort to understand the changes in neural function that precede and accompany onset of dementia and their relation to clinical manifestations, in the last several years, we have conducted clinical, neuropsychological and brain metabolic studies in groups of Alzheimer's disease patients at different stages of dementia severity or with distinct clinical pictures and in populations at risk for developing the disease. Here, we discuss the main findings and implications obtained from these studies.