This article argues that in Old English poetry a monster is a creature who inverts humanity so as to threaten society. It begins by outlining the definitions of monsters that were passed down to the Anglo-Saxons from three great authorities: Pliny, Augustine, and Isidore. Despite the availability of these authoritative definitions, however, Old English poets appear not to have used them. Instead, as can be seen in the depiction of Grendel, Grendel’s mother, and the dragon in Beowulf, monsters are distinct from both animals and human beings in their threat to society. This threat is not merely physical; in fact, its most troubling aspect is the way in which monsters parody and invert the society that attempts to exclude them. Thus t...