This is a paperback reprint which comprises research papers on the formal (mathematical) structure of natural language. In addition to older classic papers, the collection includes papers on such current topics as GPSG and modern categorial grammar. Although it addresses a number of mathematical questions and even includes some formal mathematics, the book is accessible to all linguists, even if they have only limited formal training in mathematics. Most of the papers deal with questions about the fit between formal theories of syntax and the empirical data on natural language.