This book contains a selection of papers presented at the 9th Conference on Laboratory Phonology, which was held in June 2004 at the Beckman Institute for Advanced Science and Technology on the campus of the University of Illinois at Urbana-Champaign. The theme of the conference was Change in Phonology, broadly conceived as to include both language evolution at the level of the speech community and development within the individual speaker/hearer. The chapters in this book are organized in five sections: phonological variation and change within the speech community, mechanisms of change in sound systems, phonological acquisition from different experimental perspectives, second language phonology, modeling of language variation, and segmental and suprasegmental phenomena related to the timing of speech gestures. These topics are explored from a number of perspectives, both within and outside of traditional linguistics. We believe that the papers included in this volume demonstrate that the Laboratory Phonology approach has reached maturity and has succeeded in its aim not only to bridge the gap between phonetics and phonology but also to establish a fruitful and mutually beneficial dialog between linguists and other scientists and scholars concerned with the study of the sound patterns of human language from different perspectives.