The book aims to bridge the gap between standard textbooks on the subject and more advanced books, discussing in a more thorough way the foundations of both classical and quantum statistical mechanics. It is devised for a one-semester graduate course on statistical mechanics. Its prerequisite is therefore a more elementary course on the same subject. Emphasis is laid on the geometrical aspects of classical thermodynamics, on the foundational problems, and on selected applications (mainly to spin systems), the meaning of quantum statistics, phase transitions, critical phenomena, and the renormalization group.