This book constitutes the first full-length diachronic treatment of the English progressive from Old English to Present-day English, focusing on the crucial phase of its grammaticalization between the 17th and 20th centuries. It uses data from the British component of ARCHER-2 (A Representative Corpus of Historical English Registers, version 2) to uncover the details of this long-term grammaticalization process, tracing the development of the construction from a stylistic device to a fully-fledged aspect marker. Illustrated by a wealth of examples, the work offers new results concerning the preferred linguistic environments and the development of the functions of the progressive. In contrast to previous studies, the author shows that there are certain restrictions to context expansion in grammaticalization. She argues convincingly that the persistent reluctance of the progressive to occur in certain contexts does not point to incomplete grammaticalization, but can instead be explained as a product of its particular functions. The author also challenges the tenet that grammaticalization is generally accompanied by subjectification.