Two of the best studied and most intriguing areas in syntax and semantics are ellipsis and wh-movement, and although these areas have generated immense interest individually, their intersection-in elliptical wh-questions known as sluicing-has remained largely neglected. This book fills that gap. It does so on the basis of the most sustained empirical investigation of sluicing ever conducted, drawing on novel data from thirty languages that give rise to a number of surprising and theoretically-challenging generalizations. The author shows that sluicing structures are crucial to answering the fundamental questions about the nature of ellipsis: how ellipsis is resolved, whether there is syntactic structure internal to the ellipsis site, and whether the identity requirement on ellipsis is syntactic or semantic. The author proposes a new and elegant theory of ellipsis based on semantic identity, and shows how this theory overcomes problems encountered by common alternatives based on syntactic isomorphism. He posits that ellipsis sites are syntactically complete, though unpronounced, and provides a novel account of how a semantic theory of ellipsis is compatible with syntactic deletion. The facts of sluicing argue also that our conception of islandhood must be refined in fundamental ways, leading to a pluralistic view of islands, with wh-movement extraction deviancies distributed over different components of the grammar. This work sheds new light on some of the most central and long-standing questions in the study of ellipsis and wh-movement, and has important implications for understanding the relations between syntax and semantics. Jason Merchant writes accessibly for linguists of all schools and persuasions. The issues he addresses will interest theoreticians and typologists, especially syntacticians, semanticists, and those interested in the syntax-semantics interface.