Rapid eye movement (REM) sleep behaviour disorder (RBD) is a parasomnia characterized by the intermittent loss of electromyographic atonia normally present during REM sleep and the emergence of purposeful complex motor activity associated with vivid dreams. Rapid eye movement sleep behaviour disorder usually affects older males and can be either idiopathic or symptomatic of various underlying disorders, in particular neurodegenerative diseases; in the latter case, RBD may be a prodromal symptom of the neurological disease. Several brainstem regions have been implicated in RBD pathophysiology, although the exact mechanism of the disorder in humans remains to be clarified. On clinical grounds, differentiation of RBD should be made from several non-REM parasomnias and other aberrant behaviours occurring during sleep. Rapid eye movement sleep behaviour disorder can be diagnosed on the basis of a systematic medical, neurological and psychiatric evaluation of the patient, assisted by a standard polysomnographic recording that includes continuous overnight videotaping; a brain imaging study is mandatory when an underlying brain disease is being suspected. Clonazepam at bedtime is the treatment of choice for RBD; alternatively, melatonin or pramipexole can be administered when clonazepam is contraindicated.