The subject itself has progressed considerably in recent years, especially in relation to the theory of phase changes and various aspects of the ergodic problems. In order to include recent developments of the theory of phase changes, more than half of Chap. 4 has been rewritten. It is hoped that the inclusionof additional material will elucidate the current point of view and the new methods employed in this fascinating branch of statistical physics. Chapter 5, which is devoted to the ergodic pr