The widespread occurrence of the various forms of arthritis not only results in a great waste of manpower, but also causes immeasurable pain and suffering for the patients. Due to the limited understanding of its etiology, the currently available treatments are directed at the effects of the disease rather than its causes. The solutions available to the clinician at the advanced stages of arthritis are frequently surgical and include prosthetic replacement arthroplasty. Many advances have been made in the last decade in the basic understanding of the kinematics and kinetics of anatomical joints, as well as in the technology of joint replacement. The NATO Advanced Study Institute held in Portugal during June 20-July 1, 1983 addressed these topics and provided instruction on the advances in biomechanics of diarthrodial joints. The proceedings of this Institute are presented in this volume. Many different areas of specialization contribute to the field of joint biomechanics. Due to the complexity of each individual topic, it was not attempted here to present a complete treatise of each of these areas. Each chapter typically gives a review and a flavor of the subject matter, as well as discussing the state-of the-art advances in general or in specific research areas. Some of the chapters, such as those on lubrication and muscle mechanics, are more mathematically oriented than the others. Nevertheless, the reader with a non-engineering background, I trust, would still find most of the book informative and easy to read.