REM sleep is the stage associated with vivid dream mentation, desynchronous cortical EEG, and atonia of antigravitary muscles. REM sleep behavior disorder (RBD) is characterized by the intermittent loss of REM sleep atonia and by the appearance of elaborate motor activity associated with dream mentation. The animal model of REM sleep without atonia indicates that lesions to the perilocus coeruleus disrupt the excitatory connection to the nucleus reticularis magnocellularis in the descending medullary reticular formation and disable the hyperpolarization of the alpha spinal motoneurons. Extensive neurologic evaluations in humans suffering from both idiopathic and symptomatic forms have not identified specific lesions; however, findings in some patients suggest that diffuse lesions of the hemispheres, bilateral thalamic abnormalities, or primary brain-stem lesions may result in the RBD. Symptomatic RBD cases are associated with several neurologic disorders such as dementia, cerebrovascular diseases, multiple sclerosis, brain-stem neoplasm. RBD has been often documented to precede or to co-occur with neurodegenerative disorders, such as dementia, Parkinson's disease and multiple system atrophy. Most importantly, RBD is readily diagnosable and treatable. Patients and their bed partners usually report immediate improvement in sleep-related motor behavior with small doses of clonazepam.