Positron Emission Tomography (PET) is a quantitative technique which can be used to measure regional values of cerebral metabolism, or uptake of radio-labeled tracers, in the living human. While structural scans (CT or MRI) may be normal in the degenerative dementias, or show generalized atrophy, functional imaging techniques allow differentiation of subtypes of dementias according to patterns of abnormal metabolism. This paper describes some of the variety of alterations in patterns of energy metabolism that may be observed in degenerative cognitive disorders and their correlations with clinical subtypes, together with an in vivo study of the dopaminergic system in subtypes of patients with dementia of the Alzheimer type (DAT).