Parkinsonism is most of the time caused by idiopathic Parkinson's disease (IPD). Considering the differences in therapeutic response and prognosis, in vivo discrimination between IPD and "parkinsonism-plus" syndromes is important. Recently, ligands have become available for imaging the pre- and postsynaptic dopaminergic system by Single Photon Emission Computed Tomography (SPECT). Visualization of postsynaptic D2 dopamine receptors using 123I-iodobenzamide (123I-IBZM) may contribute to the differential diagnosis between IPD and "parkinsonism-plus" syndromes as IPD is a pure presynaptic disease. Imaging of the presynaptic dopamine transporters using [123I] beta-CIT (2 beta-carbomethoxy-3 beta-(4-iodophenyl)tropane) may be used as a diagnostic technique. Early disease detection in subjects suspected to be at risk for developing IPD has become possible using [123I] beta-CIT or other ligands for the dopamine transporter. Furthermore, with SPECT one is probably able to monitor in an objective way the efficacy of new pharmacological therapies.