REM sleep behaviour disorder (RBD) is a parasomnia characterised by nocturnal complex motor activity associated with dream mentation. RBD, which affects mainly older men, may be idiopathic or associated with other neurological 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 RBD patients. Follow-up studies in small samples revealed that a proportion of RBD patients will eventually develop Parkinson's disease and/or a dementia of Lewy bodies type in the years following the RBD diagnosis. Recently, neurophysiological and neuropsychological studies in idiopathic RBD have found evidence of central nervous system dysfunction. An impairment of cortical activity, specific neuropsychological deficits, signs of autonomic dysfunction and olfactory impairment have been observed in these patients, challenging the concept of idiopathic RBD. The detection of early markers of neurodegenerative disorders in idiopathic RBD, and the evaluation of their value by the combined application in prospective studies may be crucial for developing early intervention strategies.