The study and implementation of microphone arrays originated over 20 years ago. Thanks to the research and experimental developments pursued to the present day, the field has matured to the point that array-based technology now has immediate applicability to a number of current systems and a vast potential for the improvement of existing products and the creation of future devices. In putting this book together, our goal was to provide, for the first time, a single complete reference on microphone arrays. We invited the top re­ searchers in the field to contribute articles addressing their specific topic(s) of study. The reception we received from our colleagues was quite enthusi­ astic and very encouraging. There was the general consensus that a work of this kind was well overdue. The results provided in this collection cover the current state of the art in microphone array research, development, and technological application. This text is organized into four sections which roughly follow the major areas of microphone array research today. Parts I and II are primarily the­ oretical in nature and emphasize the use of microphone arrays for speech enhancement and source localization, respectively. Part III presents a num­ ber of specific applications of array-based technology. Part IV addresses some open questions and explores the future of the field.