The phenomenon of grammaticalization - the historical process whereby new grammatical material is created - has attracted a great deal of attention within linguistics in recent years. However, until now no attempt has been made to provide a general account of this phenomenon in terms of a formal theory of syntax. The aim of this new and original book is to do precisely that. Using Chomsky's Minimalist Program for linguistic theory, Roberts and Roussou show how this approach gives rise to a number of important conceptual and theoretical issues concerning the nature of functional categories and the form of parameters, as well as the relation of both of these to language change. Drawing on examples from a wide range of languages, they construct a general account of grammaticalization with implications for linguistic theory and language acquisition.