Medieval people knew as well as we do that no two people could look exactly alike. But the question of look-alikes was nevertheless a fascinating one because it opened up a potential gap between external manifestation and deeper identity. What did it mean for the ontology of the self if one person could be substituted for another with nobody being the wiser? This article examines two medieval reflections on the possibility of an absolute resemblance that enables one character to be substituted for another. The first is the case of the “False Guenevere” in the Lancelot section of the enormous thirteenth-century Vulgate Cycle of the Arthurian romances, and the second is from Spain: El Conde Lucanor by the great fourteenth-century writer Don Juan Manuel. The two instances contrast nicely. In the first, Queen Guenevere's look-alike half-sister arrives at Arthur's court with the false claim that she is the true Guenevere and that the sitting queen has usurped her place. Indeed, she triumphs temporarily until Lancelot restores the proper order through trial by combat. In the second, an angel counterfeits a king's physiognomy (after the king commits blasphemy) and dresses in his clothes, leaving him only a pauper's vestments. In the Arthurian tale, the problem posed is how to know the true inner self when two identical-looking women both claim to be the “true” queen — a question that is complicated by the fact that the “true” Guenevere has been “false” for many years due to her adulterous affair with Lancelot. Conversely, Juan Manuel poses the question of identity as one of context: when the king is deprived of the trappings of his kingship, no one recognizes him. Both texts point up the problems of signification and identity that haunted medieval thinking. What is the relationship between one's outer appearance and inner nature? What is the relationship between a sign and what it signifies?