The pervasive phenomenon of metadiscourse – commentary on the ongoing discourse – is beginning to take its rightful place among the major topics of discourse studies. This book makes simultaneous contributions to the theory of metadiscourse, corpus-based methods of studying such phenomena, and our knowledge of metadiscourse use in written English. After comprehensively reviewing previous research, it introduces a more rigorous and empirical approach to metadiscourse studies. ƒdel presents a new model of metadiscourse based on Jakobson’s functions of language, and other conceptual tools, including explicit features for defining metadiscourse, a taxonomy of the functions it serves, and maps of the boundaries between it and related phenomena. A large-scale study of writing by L1 and L2 university students is presented, in which the L2 speakers’ overuse of metadiscourse strongly marks them as lacking in communicative competence. This work is of interest both to linguists and to educators concerned with writing in English.