Rapid eye movement (REM) sleep behavior disorder (RBD) is a parasomnia which occurs only during REM sleep. RBD is characterized by the loss of skeletal muscle atonia, which is the principal feature of REM sleep, and by abnormal behavior representing the attempted enactment of dreaming. Clinically, it consists of abnormal behavior, frequently violent, that may lead to injuries. A polysomnographic study is necessary to assess the diagnosis showing the absence of REM sleep atonia and related abnormal behavior. RBD usually affects older men. Its exact etiological factors remain unknown. However, RBD is sometimes linked with drug use or exposure to toxic conditions. Moreover, the association with neurological disorders is frequently reported, particularly cerebrovascular, tumoral or neurodegenerative disease. Neurodegenerative disorders with parkinsonism are frequently involved. In Lewy body dementia, RBD is particularly frequent. In many cases, RBD may have a predictive value, preceding other symptoms of neurodegenerative disorders with parkinsonism. The biochemical mechanisms underlying RBD still remain unclear but could involve the dopamino-cholinergic balance and the serotoninergic pathway.