Scholars are sometimes called upon to adjudicate charges of plagiarism, that is, of the unacknowledged “lifting” of one writer's words or ideas by another. When literal copying of whole expressions or sentences is involved, judgments are straightforward. But when the accuser claims that his or her ideas were plagiarized, as in the recent court case against Random House over Dan Brown's Da Vinci Code, the situation is much more complicated. Passing judgment in such cases requires a reflection on the meaning of “plagiarism” and the best ways of assessing it, which this brief article seeks to provide.