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