This volume, together with its two companion volumes, originated in a study commis- sioned by the United States National Academy of Sciences on behalf of the National Aeronautics and Space Administration. A committee composed of Tom Holzer, Dimitri Mihalas, Roger Ulrich and myself was asked to prepare a comprehensive review of current knowledge concerning the physics of the Sun. We were fortunate in being able to persuade many distinguished scientists to gather their forces for the preparation of 21 separate chapters covering not only solar physics but also relevant areas of astrophysics and solar-terrestrial relations. It proved necessary to divide the chapters into three separate volumes that cover three different aspects of solar physics. Volumes II and III are concerned with 'The Solar Atmosphere' and with 'Astrophysics and Solar-Terrestrial Relations'. This volume is devoted to 'The Solar Interior', except that the volume begins with one chapter reviewing the contents of all three volumes. Our study of the solar interior includes a review of nuclear, atomic, radiative, hydrodynamic and hydromagnetic processes, together with reviews of three areas of active current investigation: the dynamo mechanism, internal rotation and magnetic fields, and oscillations. The last topic, in particular, has emerged in recent years as one of the most exciting areas of solar research.