Rapid eye movement (REM) sleep behavior disorder (RBD) is a multifaceted motor, behavioral, and experiential disorder of REM sleep in which the affected person typically enacts in his sleep the stereotypically altered dreams that have intruded on him, featuring confrontation, aggression, and violence, but lacks any daytime tendency for increased aggressiveness. The vigorous and violent behaviors of RBD commonly result in injury, which at times can be severe and even life threatening. The core electromyographic (EMG) abnormalities of RBD consist of intermittent loss of the usual skeletal muscle atonia of REM sleep, with increased muscle tone and/or excessive phasic muscle twitching during REM sleep. RBD occurs naturally across mammalian species, and there is an experimental animal model of RBD induced by dorsal pontine tegmental lesions in cats. Apart from acute RBD that usually emerges during drug-induced states, toxic-metabolic states, and drug withdrawal states, RBD is a chronic disorder that rarely has spontaneous remissions. RBD is often the harbinger of a parkinsonian or other neurodegenerative disorder and can be regarded as a sentinel or cryptogenic disorder.