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 problems, has been fully revised to present contemporary knowledge of the ergodic behavior of mechanical systems, which has been actively investigated in the last few years by means of mathematical analysis, supported by numerical computation. The authors have also taken advantage of the opportunity to correct typographical errors, and to revise some figures.