Includes bibliographies and index.

This book treats the problem of phase transitions, emphasizing the generality and universality of the methods and models used. The course is basically concentrated on the problems of vacuum degeneration in macroscopic systems and a fundamental concept of quasiaverages by Bogolubov playing a special role in the theory of phase transitions and critical phenomena. An analysis of the connection between phase transition and spontaneous symmetry breaking in a macroscopic system allows a unique description of both first- and second-order phase transitions.