Statistical mechanics is the application of physics or chemistry at finite temperature T and can encompass as many topics involving these disciplines as one wishes. As much as the quality of presentation, it is the choice of topics that distinguishes from one another the scores of textbooks with the word "statistical" or "statistics" in the title. The present book is intended to respond to the curiosity of the reader about fundamental principles. It shows in detail how one solves the problems that arise in connection with these principles. There are some 50 problems scattered throughout the text and a similar number of illustrations. All the mathematics is self-contained, includ