 John Duns Scotus (1265/66–1308) was one of the most important and influential philosopher-theologians of the High Middle Ages. His brilliantly complex and nuanced thought, which earned him the nickname “the Subtle Doctor,” left a mark on discussions of such disparate topics as the semantics of religious language, the problem of universals, divine illumination, and the nature of human freedom. This essay first lays out what is known about Scotus’s life and the dating of his works. It then offers an overview of some of his key positions in four main areas of philosophy: natural theology, metaphysics, the theory of knowledge, and ethics and moral psychology.