This book describes the historical emergence and spread of the to-infinitive in English. It shows that to + infinitive emerged from a reanalysis of the preposition to plus a deverbal nominalization, which spread first to purpose clauses, then to other nonfinite environments. The book challenges the traditional reasoning that infinitives must have been nouns in Old English because they inflected for dative case and can follow prepositions. Dr Los shows that, even as early as Old English, the to-infinitive was established in most of the environments in which it is found today. She argues that its spread was largely due to competition with subjunctive that-clauses, which it gradually replaced. Later chapters consider Middle English developments. The author provides a measured evaluation of the evidence that to undergoes a period of degrammaticalization. She concludes that the extent to which to gains syntactic freedom in Middle English is due to the fact that speakers began to equate it with the modal verbs and therefore to treat it syntactically as a modal verb. The exposition is clear and does not assume an up-to-date knowledge of generative theory. The book will appeal to the wide spectrum of scholars interested in the transformation from Old to Middle English, as well as those studying the processes and causes of syntactic change more generally.