This book is the first detailed description of a particular theoretical framework for studying language development and language performance. The framework is called MOGUL (Modular On-line Growth and Use of Language). It has been the topic of numerous publications and presentations since the appearance of our 2004 keynote article in Bilingualism: Language and Cognition. MOGUL is not just about how a language grows in the individual child: it is about how the mind expands to accommodate more than one language both in childhood and later in life and how these various linguistic systems share space and interact.

The intended audience is interdisciplinary so the more technical details of, for example linguistic theory, have been avoided where possible or explained so that the book can appeal to a wider audience interested in language, bilingualism and language acquisition including specialists and students at graduate level in linguistics, psycholinguistics, neurolinguistics, and cognitive psychology. A detailed MOGUL glossary is currently available at the following website: www.cambridge.org/sharwoodsmith_truscott.

This book draws together insights from a number of related fields in cognitive science to provide an overall cross-disciplinary ‘big picture’ perspective within which research in separate research domains can be viewed and re-evaluated. In demonstrating how the framework may be used it also makes a number of specific claims about the growth and use of languages.

The work of Ray Jackendoff has been a major inspiration and starting point, although his views are developed and interpreted here in a number of ways peculiar to the MOGUL framework. Our aim has been to set out in some detail what may be the first explicit representational account of exactly how languages develop in the mind of an individual millisecond by millisecond as well as month by month in response to exposure to utterances in the environment. It is an on-line processing-based account that is broadly compatible with logical explanations in the generative linguistics literature which focus on the properties of developmental grammars at different stages and not how they actually change over time. In other words, this is a symbolic representational account as well as a processing one.

MOGUL, as set out in this book, is definitely unfinished business. Its future depends in large measure on its usefulness to the disciplines to which it seeks to contribute. To the extent that it has already got somewhere interesting, we can attribute its value to a mixture of serendipity and the valuable help we have received along the way from friends, students, colleagues, and also anonymous reviewers both of this book and of the various MOGUL-related publications that have appeared since 2004. In particular, we are grateful for the help and encouragement we have received from many friends, colleagues, and students, including members of the Developmental Linguistics group at Edinburgh University, but particularly to Harald Clahsen, Ray Jackendoff, Donna Lardiere, William O’Grady, Monika Schmid, Ianthi Tsimpli, Paul van Buren, Melinda Whong, and Bill VanPatten. In all fairness we should include in our acknowledgements the inspirational city of Edinburgh itself, which might well be regarded as the home of MOGUL (MOGUL's midwife!), but not forgetting its MOGUL sister, Hsinchu in Taiwan, and the much travelled digital highway that links these two cities. Much of the work on this book was done during two research leaves that Truscott spent in Edinburgh, allowing us to go beyond the inevitable limitations of the digital highway. We wish to thank National Tsing Hua University for these opportunities. Last but definitely not least very special thanks are due to Ewa for her support and infinite patience with this long-lasting and time-consuming project.