The goal of this book is to describe and explain intrasentential codeswitching - the production of two or more languages within the same sentence. Most linquists who do not study codeswitching think of it as belonging strictly in the domain of sociolinguistics. Most codeswitching studies do indeed have a social aspect, because they typically use naturally occurring performance data as their base. However, this book is just as much a study in grammatical theory as a study of language in use. The specific research question addressed is this: when speakers alternate between two or more linguistic varieties, how free is this alternation from the structural point of view? Carol Myers-Scotton develops a model of the morphosyntactic constraints on codeswitching; she concludes that the principles governing codeswitching are the same everywhere. Her model supports a lexically based model of language production.