Functional imaging studies utilizing single-photon emission computed tomography (SPECT) or positron emission tomography (PET) have garnered increasing attention in neurology, particularly in Parkinson’s disease (PD) and other neurodegenerative disorders. These imaging techniques have proven to be important for diagnosis and understanding disease mechanisms. In this chapter, we aim to explore the principles of molecular imaging in PD, as well as its limitations. Also, it is essential to view PET and MRI as complementary rather than competing approaches in addressing these complex questions.