From 1938 to 1940 Karl Llewellyn worked on and substantially completed a book-length manuscript entitled The Theory of Rules. With the book almost completed, Llewellyn turned to other things, primarily his work on law and anthropology and his efforts relating to the Uniform Commercial Code. The book on rules remained uncompleted at his death, and is now to be published by the University of Chicago Press in 2010, edited and with an Introduction by Frederick Schauer, David and Mary Harrison Distinguished Professor of Law at the University of Virginia. The Introduction situates The Theory of Rules within the corpus of Llewellyn’s work, within the Legal Realist perspective on legal rules, and within the jurisprudential and philosophical literature on rules in general.