Book excerpt: The importance of phonons has long been recognized by research ers in nonequilibrium superconductivity. Similarly, experimentalists studying phonons at low temperatures have relied heavily on supercon ductors as sources and detectors. To a large extent this symbiotic relationship has developed with a general mutual awareness; however, to our knowledge these subjects have never been treated together in conferences or study institutes. It was with the hope of further contributing to the awareness and communication between workers in these areas that this NATO Advanced Study Institute (ASI) has been conceived. A second, but equally important, reason for holding this ASI is to fill a void by providing the first general textbook in this important area of physics. Therefore, there was an emphasis on the tutorial nature of the lectures and written contributions to this textbook. It should not go unnoticed that the experimental and theoretical concepts covered in this textbook are of paramount importance to the various applications of superconductors. Almost by definition, the use of a superconductor implies a nonequilibrium state! For example, phonon conduction to the helium bath is important in devices ranging from microscopic Josephson junctions to large scale magnets and transmission lines. Knowledge of the more fundamental nonequilibrium effects can aid in our understanding of devices as well as provide the potential for entirely new applications.