Intelligibility is the sine qua non of spoken language and disorders that impair intelligibility are among the most serious disorders of communication. The papers in this volume, written by authors experienced in intelligibility issues in speech pathology and related fields, describe the basic dimensions by which speech intelligibility can and must be understood. The dimensions are auditory perceptual, linguistic, acoustic and physiologic. These, in turn, are applied to the fundamental problems of definition and theory, measurement and clinical management.