In this review article the significance of molecular imaging techniques, single photon emission tomography and positron emission tomography (SPET and PET), in the diagnosis and differential diagnosis of various types of dementia is described. A major part of this article is focused on molecular imaging in Alzheimer's disease where parietal, temporal and posterior cingulate cortex hypoperfusion and hypometabolism are the predominant findings in molecular imaging modalities. In cases of mild amnestic cognitive impairment, molecular SPET imaging can differentiate patients converting to Alzheimer's disease from non converters. Molecular SPET imaging with pre-synaptic dopamine receptors radioligand ((123)I-ioflupane or DaTSCAN), is the method of choice in order to differentiate Alzheimer's disease from Lewy body dementia. Finally, nuclear medicine procedures support the diagnosis in fronto-temporal and multi-infarct dementia.