Auxiliaries form one of the most complex areas of English syntax. Disagreement over both the principles and the details of their grammar has been substantial. Anthony Warner offers a new and detailed account of both their synchronic and their diachronic properties. He first argues that lexical properties are central to their grammar, and gives a brief account within Head-driven Phrase Structure Grammar. He then traces in detail the history of processes of grammaticalization in their development, claiming that we can identify a group of auxiliaries on formal grounds from an early period, and working out the consequences of this view for their status as a word class. The account ties together the establishment of the class of auxiliaries with loss of the general verb-second order of earlier English, and interprets both as factors underlying the development of the modern use of do . This book meets the dual challenge of accounting for both the grammar and the history of the English auxiliary. It will be essential reading for all those interested in English syntax and its history.