Rapid-eye movement (REM) sleep behaviour disorder (RBD) is an REM sleep parasomnia characterized by enactment of dream content during REM sleep associated with loss of muscle atonia. RBD can be either idiopathic or secondary to drugs or other diseases. The best recognized association is with neurodegenerative diseases, namely alpha-synucleinopathies. RBD may represent the first feature of neurodegeneration and can be considered an early marker of these disorders. This review describes the main clinical, pathogenetic, and therapeutic features of RBD, pointing to its association with neurodegenerative diseases and emphasizing the clinical and prognostic implications.