REM sleep behaviour disorder (RBD) is an important parasomnia characterized by the intermittent loss of physiologic REM sleep muscle atonia and the appearance of abnormal, frequently violent, complex movements during sleep. This motor activity is associated with dream mentation and may lead to the injuries of the patient and/or of the persons in the bedroom. The exact etiological factors of RBD remain unknown. RBD can be misdiagnosed as a psychiatric disorder. It usually affects middle-aged or older men and may present itself as an acute illness after overdosed or suddenly withheld medications (e.g. antidepressants), and more frequently as a chronic disease. In its chronic form it is usually followed by the development of neurodegenerative diseases, especially by Parkinson's disease. The association of RBD with narcolepsy or cerebrovascular diseases is also common. Very rare coincidence with obstructive sleep apnea syndrome may indicate its protective action against upper airway collapse during sleep. A polysomnographic study is necessary to assess the diagnosis of RBD showing the absence of REM sleep atonia and related abnormal behaviour. Clonazepam, pramipexol, and melatonine have been tried in the treatment of RBD.