Rapid eye movement (REM) sleep behavior disorder (RBD) is a sleep disorder characterized by enacting one's dreams during the REM sleep, with most of the dreams being violent or aggressive, so that patients often come to see the doctor complaining hurting themselves or bed partners during sleep. Prevalence of RBD, based on population, is 0.38-2.01 %, but much higher in patients with neurodegenerative diseases, especially synucleinopathies. RBD may herald the emergence of synucleinopathies by decades, such that it may be used as an effective early marker of neurodegenerative diseases. Pharmaceutical treatment of RBD includes clonazepam, melatonin, pramipexole, and some newly reported medications. In this review, we summarized the clinical and PSG features of RBD, the pathophysiology and the therapy of it, focusing on the correlation between neurodegenerative diseases and RBD, in order to emphasize the significance of RBD as an early marker of neurodegenerative diseases.