The increasing availability of PET imaging in nuclear medicine expands the armamentarium of clinical and research tools for improving diagnosis and treatment of neuropsychiatric disorders. Nonetheless, the role of SPECT imaging remains critical to both research and clinical practice. The development of rational strategies for guiding the selection of imaging modalities flows from primarily the nature of the clinical or research question and the availability of appropriate radiopharmaceuticals. There has been extensive SPECT and PET work in Parkinson's disease (PD) which highlights the value of both these scintigraphic modalities. Three main areas of interest in PD include imaging for improving diagnostic accuracy, for monitoring the progression of disease, and for assessing the therapeutic efficacy of drugs with neuroprotective potential. The demands of the clinical or research question posed to imaging dictates the selection of radiotracer and imaging modality. Diagnosis of PD represents the easiest challenge with many imaging biomarkers showing high sensitivity for detecting abnormal reduction of dopaminergic function based on qualitative review of images. On the other hand, using imaging to evaluate treatments which purportedly slow the rate of disease progression, indicated by the reduction in the rate of loss in a quantitative imaging signal in patients studied over time, represents the most rigorous requirement of the imaging measure. In each of these applications presynaptic markers of dopaminergic function using SPECT and PET have been extremely valuable. Review of neuroimaging studies of PD provides a useful example of optimized approaches to clinical and research studies in neuropsychiatric disorders.