This book presents a critical and modern analysis of the conceptual foundations of statistical mechanics as laid down in Boltzmann's works. The author emphasizes the relation between microscopic reversibility and macroscopic irreversibility. Students will find a clear and detailed explanation of fundamental concepts such as equipartition, entropy, and ergodicity. They will learn about Brownian motion, the modern treatment of the thermodynamic limit phase transitions, the microscopic and macroscopic theory of the coexistence of phases, statistical mechanics of stationary states, and fluctuations and dissipation in chaotic motions.