This is the first detailed study to explore the little-understood notions of "knowing who someone is," "knowing a person's identity," and related locutions. It locates these notions within the context of a general theory of believing and a semantical theory of belief- and knowledge-ascriptions.The books's main contention is that what one knows, when one knows who someone is, is not normally an identity in the numerical sense of "a = b," but rather a certain sort of predication to know who someone is is just to know that that person is F, where "F" is a predicate that is "important," in a technical sense defined by the authors, for the purposes determined by context. Their book offers a rigorous formal semantics for ascriptions of knowing and of knowing-who in particular, solving such well-known problems and paradoxes as Kripke's Puzzle, and Quines difficulties with de re belief, along the way.The authors apply their analysis to each of several important issues in philosophy of language, philosophy of mind, and ethical theory in which the previously unexamined notion of "knowing who" has loomed large-the mechanics of linguistic referring, the foundations of epistemic logic, problems of self-knowledge and self-regarding belief, universalizability and "Golden Rule" arguments in ethics, and moral "personalism" versus "impartialism. "Stephen Boer is Professor of Philosophy at the Ohio State University. William Lycan is Professor of Philosophy at the University of North Carolina and author of Logical Form in Natural Language (Bradford Books, 1984). A Bradford Book.