This book consists of three parts. Part I, which includes Chapters 1-4, gives the principles, with elementary applications to noninteracting systems. It begins with kinetic theory and discusses classical and quantum systems in equilibrium and nonequilibrium. Examples in this part do not directly deal with interactions. Nevertheless, an overall picture of statistical mechanics may be obtained here at an elementary level. In Part II, which consists of Chapters 5-9, classical statistical mechanics is developed for interacting systems in equilibrium and nonequilibrium. Finally, in Part III, quantum statistics is presented to an extent which enables the reader to proceed to advanced many-body theories.