The accurate diagnosis of idiopathic Parkinson's disease (IPD) is not only important for deciding on treatment strategies and providing a prognosis, but also crucial for studies designed to investigate the aetiology and pathogenesis of parkinsonian disorders. Over recent decades, improvements in the characterisation of the parkinsonian syndromes have led to improvements in clinical diagnostic accuracy; however, clinical criteria alone are not always sufficient to distinguish between IPD and other parkinsonian syndromes, particularly in the early stages of disease and in atypical presentations. Therefore, in addition to the development and implementation of diagnostic clinical assessments, there is a need for available objective markers to aid the physician in the differential diagnosis of IPD. Functional neuroimaging holds the promise of improved diagnosis and allows assessment in early disease. In this review, the use of PET and single photon emission CT in the differential diagnosis of IPD are discussed.