This book is a practical guide for researchers and advanced graduate students in biology and biophysics who need a quantitative understanding of acoustical systems such as hearing, sound production, and vibration detection in animals at the physiological level. It begins with an introduction to physical acoustics, covering the fundamental concepts and showing how they can be applied quantitatively to understand auditory and sound-producing systems in animals. Only after the relatively simple mechanical part of the system is explained does the author focus his attention on the underlying physiological processes. The book is written on three levels. For those wanting a brief survey of the field, each chapter begins with a nonmathematical synopsis which summarizes the content and refers to the figures, all of which are designed to be understood apart from the main text. At the next level, the reader can follow the main text, but need not give close attention to anything but the general concepts and techniques involved. At the third level, the reader should follow the mathematical arguments in detail and attempt the discussion of questions at the end of each chapter. The author has provided detailed solutions which serve to expand the discussions of particular cases.