Exploring the concept of the monster in the Middle Ages, this work examines its philosophical and theological roots, and analyzes its symbolic function in medieval literature and art. Part I traces the poetics of teratology, the study of monsters, to Christian neoplatonic theology and philosophy, particularly Pseudo-Dionysius's negative theology and his central idea that God cannot be known except by knowing what he is not. The author argues that the principles of negative theology as applied to epistemology and language made possible a symbolism of negation and paradox whose chief sign was the monster. Part II provides a taxonomy of monstrous forms with a gloss on each, and Part III examines the monstrous and the deformed in three heroic sagas - the medieval "Oedipus", "The romance of Alexander", and "Sir Gawain and the Green Knight" - and three saints' lives - Saint Denis, Saint Christopher, and Saint Wilgeforte. The book is illustrated with medieval representations of monsters. A comprehensive study of the grotesque in medieval aesthetic expression, this work brings together medieval research and modern criticism.