The three states of mammalian being-wakefulness, REM sleep, and NREM sleep-are not mutually exclusive and may occur simultaneously, oscillate rapidly, or appear in dissociated or incomplete form to produce primary sleep parasomnias. Dysfunctions of a wide variety of organ systems may take advantage of the sleeping state to declare themselves, resulting in the secondary sleep parasomnias. Contrary to popular opinion, most of these often bizarre and frightening experiences are not the manifestation of underlying psychological or psychiatric conditions. Formal study in an experienced sleep disorders center usually reveals a diagnosable and treatable condition. Various parasomnias may result in injurious or violent behavior. The forensic science implications are beyond the scope of this article but have been reviewed extensively elsewhere. Continued study of unusual sleep-related events undoubtedly will reveal more fascinating conditions, expanding our knowledge of sleep physiology and strengthening the bonds between clinicians and basic science sleep researchers.