REM sleep behavior disorder (RBD) is a parasomnia characterized by dream-enacting behaviors, nightmares, and REM sleep with increased muscle activity. The pathophysiology of RBD is associated with the dysfunction of the brainstem nuclei (magnocellularis nucleus in the medulla and subcoeruleus nucleus in the pons) and their connections with the basal ganglia and limbic system. RBD can be classified into an idiopathic form and a secondary form. Most patients with idiopathic RBD will eventually be diagnosed with the synucleinopathies Parkinson disease, dementia with Lewy bodies, and multiple system atrophy. Patients with idiopathic RBD show subclinical abnormalities that are characteristic of these neurodegenerative disorders such as substantia nigra hyperechogenicity on transcranial sonography, decreased putaminal dopamine binding on functional neuroimaging, smell loss, and constipation. Deposits of synuclein may be detected in the colon and salivary glands of patients with idiopathic RBD. Subjects with idiopathic RBD are optimal candidates to test disease-modifying strategies to stop the development of parkinsonism and dementia. The secondary form of REM sleep behavior disorder occurs in patients already diagnosed with neurodegenerative diseases such as Parkinson disease (25–58 % of the cases), dementia with Lewy bodies (70–80 %), and multiple system atrophy (90–100 %), and other conditions like brainstem lesions and narcolepsy. Clonazepam (0.25–4 mg) and melatonin (3–12 mg) at bed time are the treatments of choice to decrease the severity and frequency of nightmares and sleep behaviors.