The classification of dopamine receptors and their neuroanatomical distribution is reviewed, including the newly discovered D3, D4 and D5 subtypes. In vivo imaging techniques and methods for quantification are briefly described and PET and SPECT studies of D2 receptors in schizophrenia, under neuroleptic treatment, in aging, Parkinson's disease, Huntington's disease, tardive dyskinesia and multisystem atrophies are reviewed and compared with our own results. A short description is given of imaging studies of D1 receptors.