REM sleep behavior disorder (RBD) is characterized by the appearance of somnambulism-like behavior associated with dream mentation, and the intermittent loss of muscle atonia during REM sleep (stage 1-REM with tonic EMG; stage 1-REM). RBD is caused symptomatically or idiopathically, and its manifestation of symptoms sometimes associated with psychosocial stress. Muscle tone is often augmented during impared REM sleep observed in RBD patients. Animal experimental studies suggest that dysfunction or lesions of muscle inhibitory system in brain stem is rerated with pathogenesis of RBD. Clonazepam is effective for organic behavior and nightmare, but not remarkable for dissociated REM sleep. Other benzodiazepines and tricyclic antidepressants can be one of the choices of medications. Directions for safe-bedroom and psychotherapy is sometimes necessary.