Parkinson's disease (PD) is misdiagnosed throughout its disease course for conditions such as essential tremor, drug-induced parkinsonism, vascular pseudo-parkinsonism, Alzheimer's disease and other degenerative parkinsonian diseases. This thesis aims to verify the accuracy of dopaminergic imaging in early and uncertain parkinsonian/tremor disorders through 3 studies. The first is a prospective United Kingdom multicentre assessment of [1231] FP-CIT SPECT use in 190 patients in pre-defined diagnostic categories and with particular focus on clinical features to assess the influence of imaging in routine practice. The second is a 2 year follow-up study of 150 consecutive patients with normal SPECT, with specific attention to clinical progression and antiparkinson medication use, and includes focus on a subgroup who fulfilled PD criteria where successful antiparkinson medication withdrawal was achieved. The third is a multicentre prospective European study of the accuracy of [1231] FP-CIT SPECT in 99 patients that included serial clinical and imaging assessments. Notably, when initial diagnosis/scan mismatch cases occurred, and with awareness of the scan result, the clinician invariably changed the diagnosis in line with the scan result which confirms the considerable influence of imaging on the practising clinician. Parkinson's disease is clinically overdiagnosed early in its disease course, whereas imaging is more specific, in the vast majority of cases with normal dopaminergic imaging, there was no evidence of clinical or imaging progression which would be in keeping with degenerative parkinsonism.