Throughout the late medieval period (1375-1500), Middle English writers tested the capabilities of their vernacular, experimenting with new genres and styles of literary composition, as well as with discursive conventions and practices borrowed from nonliterary fields. “Subtle Arts: Practical Science and Middle English Literature” describes how, in this experimental moment, the language and rhetoric of fields of scientific practice furnished these writers with new vernacular modes of eloquence and artful language. I recover these modes of expert expression in medicine, alchemy, and astronomy from the archives of medieval life—Latin and vernacular scientific texts, as well as legal and civic records—to show how Middle English writers invoked a repertoire of language practice spoken by the physician at the bedside, the apothecary in the market, the alchemists around the furnace. Rather than regard medieval science as esoteric and ancillary to Middle English literature, “Subtle Arts” traces how these fields’ conventions of rhetoric and speech became adaptable materials for literary practice. In chapters examining the rhetoric of the Latin scientific prologue, doctors’ conversation, alchemists’ confidence routines, and proclamations of medical expertise, I argue that Chaucer, Gower, and other Middle English writers exploited modes of practical-scientific profession to craft their own performances of vernacular expertise as translators and poets, posing as authentic authorities or artful impersonators themselves.