This chapter summarizes findings from structural and functional neuroimaging studies in individuals with idiopathic rapid eye movement (REM) sleep behavior disorder (iRBD). In humans, functional brain imaging has been used to evaluate the neural mechanisms and structures that regulate REM sleep. Dysfunction of the REM sleep atonia network forms the neuropathological basis for RBD. In humans, iRBD is diagnosed when a patient with videopolysomnographic confirmation of REM sleep without atonia has no evidence of a neurological disease or other possible causes. The chapter summarizes the neuroimaging abnormalities that have been identified in Parkinson disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA). Several neuroimaging studies have evaluated the morphological and functional state of the substantia nigra in iRBD. These studies have evaluated the echogenicity of the substantia nigra with transcranial sonography (TCS) and the nigrostriatal dopaminergic system with functional imaging.