Alzheimer Disease (AD) is a progressive neurodegenerative disorder characterized by amyloid-β deposition, tau pathology, synaptic dysfunction, neuronal injury and cerebral atrophy. Positron Emission Tomography (PET) has transformed AD evaluation by enabling in-vivo visualization of molecular and metabolic abnormalities. 18F-Fluorodeoxyglucose (FDG) PET evaluates cerebral glucose metabolism, amyloid PET identifies fibrillar amyloid-β and tau PET provides information about neurofibrillary pathology. This review examines diagnostic and prognostic applications of PET in AD, particularly in mild cognitive impairment, atypical dementia, differential diagnosis, disease staging, progression prediction and treatment selection. Evidence indicates that amyloid PET is highly useful for establishing amyloid pathology, whereas tau PET is more closely associated with disease stage and clinical severity. FDG PET remains valuable for identifying patterns of neurodegeneration and distinguishing AD from other dementias. With the emergence of disease-modifying anti-amyloid therapies, molecular confirmation has become increasingly relevant to treatment decisions. Important limitations include cost, availability, radiation exposure, tracer-specific limitations, mixed pathology and interpretation variability. Future integration of PET with MRI, blood biomarkers and artificial intelligence may improve individualized diagnosis, prognosis and therapeutic monitoring.