Molecular imaging with various biomarkers plays a significant role in the diagnostic assessment of patients with dementing disorders, by detecting functional and pathophysiologic changes in the brain. PET imaging, in particular, allows the measurements of different molecular targets, such as functional markers of neurodegeneration (glucose metabolism), markers for pathological aggregates (such as amyloid, tau and synuclein aggregates) and neuroinflammation, and also markers of several neurotransmission systems. In the current view, neurodegenerative diseases are associated with the development of pathologic changes that play a different role in disease onset and evolution, often starting long before the onset of clinically detectable impairment. Molecular imaging represents thus a unique tool to collect relevant information contributing to accurate diagnosis, treatment and response monitoring. In this article, we review the current literature focusing on the PET radiotracers used for molecular imaging in dementia.