REM sleep behavior disorder (RBD) is a parasomnia characterized by a lack of motor inhibition during REM sleep leading to potentially harmful dream-enacting behaviors. RBD affects mainly older men and its prevalence in the general population is estimated to be around 0.5%. RBD may be idiopathic or associated with other neurologic disorders. A strong association between RBD and alpha-synucleinopathies has been recently observed, with the parasomnia often heralding the clinical onset of the neurodegenerative disease. The idiopathic form accounts for up to 60% of the cases reported in the three largest series of patients with RBD. Small clinical follow-up studies revealed that a proportion of these patients will eventually develop a parkinsonian syndrome or a dementia of Lewy bodies type in the years following the RBD diagnosis, while some patients will never show other neurologic signs within several decades from the RBD onset. Recent studies have looked at neurophysiologic and neuropsychological functions in idiopathic RBD and have found evidences of CNS dysfunction during both wakefulness and sleep. An impairment of the cortical activity, specific neuropsychological deficits, and signs of autonomic dysfunction have been observed in a variable proportion of these patients, challenging the concept of idiopathic RBD. Identifying subjects with a high risk of developing a neurodegenerative process may be crucial in order to develop early intervention strategies.