Sleep disorders have a high prevalence in patients with Parkinson's disease--some authors report it to be in the range of 60% - 98%. Together with the underlying motor symptoms, sleep disorders are the main causes of disability and have a substantial impact on the quality of life of these patients. Of particular interest are the behavior disorders of REM sleep (RBD) which are reported in many cases to precede the development of Parkinson's disease. In cases of diagnosing a REM sleep behavior disorder, it is absolutely necessary to exclude any underlying neurodegenerative process. Unlike the diagnosis of idiopathic RBD which can easily be made by conducting only a structured clinical interview, more than half of the RBD cases in patients with Parkinson's disease would be omitted using this technique. Patients with Parkinson's disease should be examined by polysomnography as the clinical interview's sensitivity alone can hardly reach 33%. This is so because there are mild forms of RBD in Parkinson's disease while the idiopathic forms always present with markedly severe clinical manifestations. Pathogenetically, Parkinson's disease share many similar features with RBD. Both conditions are characterized by a reduced striatal dopaminergic mediation. And yet there is no definitive answer to the question why RBD does not develop in all patients with Parkinson's disease. Clonazepam is highly effective in the treatment of RBD. Early diagnosis is thus critical for the prevention of injuries to the patient or to the patient's bed partner.