This chapter describes the pathogenesis, neurovegetative and olfactory functions, EEG activity, neuropsychological assessment, and treatment of rapid eye movement (REM) sleep behavior disorder (RBD). In RBD patient's polysomnographic (PSG) recording reveals intermittent or complete loss of REM sleep muscle atonia and excessive phasic electromyographic (EMG) activity during REM sleep. Multiple neural substrates, mainly located in the brainstem, contribute to REM sleep atonia and may be involved in the pathogenesis of RBD. Recent studies have examined various neurophysiological and neuropsychological functions in idiopathic RBD, in order to detect early signs of central nervous system (CNS) dysfunction associated with the REM sleep motor dyscontrol. Lewy body pathology begins in the anterior olfactory nucleus and in the lower brainstem nuclei, affecting olfactory and autonomic functions initially, and progressing rostrally to ultimately affect the cerebral cortex. Clonazepam is currently regarded as the treatment of choice for RBD, and it is ineff ective in only 10% of patients.