Parasomnias are defined as unpleasant or undesirable behavioral or experiential phenomena that occur predominately or exclusively during the sleep period. Initially thought to represent a unitary phenomenon, often attributed to psychiatric disease, it is now clear that parasomnias are not a unitary phenomenon but rather are the manifestation of a wide variety of completely different conditions, most of which are diagnosable and treatable. The parasomnias may be conveniently categorized as "primary sleep parasomnias" (disorders of the sleep states per se) and "secondary sleep parasomnias" (disorders of other organ systems, which manifest themselves during sleep). The primary sleep parasomnias can be classified according to the sleep state of origin: rapid eye movement (REM) sleep, non-REM (NREM) sleep, or miscellaneous (i.e., those not respecting sleep state). The secondary sleep parasomnias can be further classified by the organ system involved. The underlying pathophysiology of many parasomnias is state dissociation-the brain is partially awake and partially asleep. The result of this mixed state of being is that the brain is awake enough to perform very complex and often protracted motor and/or verbal behaviors but asleep enough not to have conscious awareness of, or responsibility for, these behaviors.