REM sleep behavior disorder (RBD), formally recognized in the 1980s, is an infrequent but spectacular parasomnia characterized by dream enactment, leading to aggressive or complex behaviors. It essentially affects older men, and many years may go by before medical attention is sought. Frequent association with the ⍺-synucleinopathies,-such as Parkinson disease, has led investigators to believe that idiopathic RBD is not necessarily a benign condition. Diagnosis is confirmed with a polysomnogram, which typically shows absence of normal REM sleep atonia and sometimes abnormal behavior. Treatment is achieved with clonazepam.