Every poet arrives at some sense of how Language works. Chaucer's engagement, like that of the greatest literary figures, goes beyond the brilliant, skilful use of Language as a tool of expression, beyond what we usually call "talent." He brings to the creative use of signification a sophisticated philosophical questioning of the very nature of Language, of how we know and how we signify. Chaucer and Language argues that Chaucer's work points to answers to these questions, emphasizing that in various ways Chaucer made Language itself the Subject of his writing. The polyvalent nature of signs and the ambiguity this makes possible are discussed as one aspect of Chaucer's use of Language as Subject, as is irony. Chaucer's extension of the concept of Language to include relics and the Eucharist, his exploitation of equivocation and the lie, and the semiotic dimensions of his poetic themes are also treated. These issues derive directly from the long tradition of mediaeval sign theory and anticipate the major issues of the modern theory of signs that is semantics.