Legal doctrine is the currency of the law. In many respects, doctrine is the law, at least as it comes from courts. Judicial opinions create the rules or standards that comprise legal doctrine. Yet the nature and effect of legal doctrine has been woefully understudied. Researchers from the legal academy and from political science departments have conducted extensive research on the law, but they have largely ignored the others' efforts. Part of the reason for this unfortunate disconnect is that neither has effectively come to grips with the descriptive meaning of legal doctrine. In this article, we attempt to describe the concept of legal doctrine and propound various theories of how legal doctrine may matter in judicial decision making and how those theories may be empirically tested.