Auxiliaries are one of the most complex areas of English syntax. Disagreement over both the principles and details of their grammar has been substantial. Anthony Warner here offers a detailed account of both their synchronic and diachronic properties. He first argues that lexical properties are central to their grammar, which is relatively non-abstract. He then traces in detail the history of processes of grammaticalisation in their development and claims most notably that we can identify a group of auxiliaries in English from an early period on formal, not just semantic, grounds. 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.