Positron emission tomography (PET) and single photon emission tomography (SPECT) are the most sensitive techniques to study, in vivo, neurotransmitter systems in normal human brain and in diseases. These methods have been applied over the last years to explore the integrity of cholinergic, serotonergic, GABAergic, and dopaminergic systems in Alzheimer's disease (AD) and in other dementias. The results of these investigations have provided new insights into the neurotransmission involvement in these disorders. Moreover, these methods provide objective quantitative markers for monitoring the progression of disease and the effects of therapies. The pathophysiological and clinical relevance of results from these studies are reviewed and the potential role in early and differential diagnosis discussed. New emerging radiotracers/methods for future investigations of neurochemical processes are delineated.