Current accepted clinical criteria for the diagnosis of Parkinson's disease (PD) provide high sensitivity for detecting parkinsonism but generally show poor specificity for identifying brainstem Lewy body disease. Biochemical markers that can be used to reliably diagnose clinical and preclinical PD have thus far been sought unsuccessfully. It is now known that some PD kindreds have a mutation of the alpha-synuclein gene, but this cannot be used as a genetic marker for most familial and sporadic cases. Functional imaging provides a means of discriminating typical from atypical PD, revealing characteristic patterns of loss of dopaminergic function. In addition, PET and SPECT show preserved levels of striatal metabolism and dopamine receptor binding in PD, whereas levels are reduced in the atypical variants. [18F]Dopa PET can also detect preclinical PD. In one series there was a reported 40% prevalence of preclinical dopaminergic dysfunction in asymptomatic adult relatives of familial PD patients. Finally, PET and SPECT can both be used to follow PD progression objectively. Such studies suggest an annual 4 to 12% loss of dopamine terminal function in early PD and a preclinical disease window of only a few years. In the future, functional imaging is likely to play an increasingly important role in assessing the efficacy of putative neuroprotective agents.