Early detection of Alzheimer’s disease (AD) is important to reveal preclinical pathological alterations, to monitor disease progression, and to evaluate response to therapy. The study of cerebral glucose metabolism through 18F-fluoro-deoxy-glucose positron emission tomography (FDG-PET) plays a leading role in early detection of AD because the decrease of cerebral glucose metabolism largely precedes the onset of AD symptoms. This technique demonstrated high sensitivity in early diagnosis of AD, allowing a qualitative and quantitative estimate of cerebral glucose metabolism. Furthermore FDG-PET imaging may help to discriminate the subjects of a high-risk population (like patients with mild cognitive impairment) who more probably will develop AD: an early stage of AD generally shows hypometabolism of medial temporal lobes and parietotemporal posterior cortices; other cerebral cortices are later involved. The combination of FDG-PET with other biomarkers, such as genotype, cerebrospinal fluid markers, and amyloid plaque imaging, may increase the preclinical diagnostic accuracy and offer promising approaches to assess individual prognosis in AD patients.