Rapid eye movement (REM) sleep behavior disorder (RBD) is characterized by dream-enacted behaviors with potential risk for injury to the patient and the bed partner. RBD is classified as a parasomnia in the International Classification of Sleep Disorders. RBD has attracted the attention of sleep researchers and clinicians for three reasons. First, it provides insight about the mechanism of REM-sleep generation. Second, it affects quality of life with social embarrassment, psychological conflict, and possibility of injuries. Third, most RBD is associated with various neurologic disorders, especially with neurodegenerative synucleinopathic diseases, including Parkinson disease, dementia with Lewy bodies, and multiple-system atrophy.