Abnormalities of REM sleep i.e. absence of atonia and abnormal motor control which are described in patients with REM sleep behavior disorder, have induced clinical and physiopathological interest. Initial hypothesis of a simple dysfunction of REM sleep generators has not been confirmed. The prevalence in older subjects and in patients with parkinsonian syndromes suggest that the appearance of the parasomnia is related to alteration in the neurons of pontine area i.e. nucleus peduncolopontine, in which there is an integration of sleep-wake mechanisms and locomotor systems. Controversy surrounds the specificity of absence of atonia in the diagnosis of the parasomnia. Since this polygraphic finding may be present in subjects without any sleep complaint and in patients with initial neurological impairment, diagnosis of the sleep disorders might be allowed only when the polygraphic characteristic of persistent tonus and complex motor behavior are recorded.