The theory of statistical mechanics is the best link we have between the imperceptible world of atoms and molecules and our common macroscopic experience. This textbook provides the fundamental rules and relationships of statistical mechanics. Through it, students will learn how to deduce the properties of materials from an underlying understanding of the behaviour of its constituent building blocks. The textbook covers the basics of systems at rest, as well as those directly manipulated. The former, also known as equilibrium statistical mechanics, is reviewed in the context of recent results in probability theory, with emphasis on solvation phenomena and phase transitions. The latter, nonequilibrium statistical mechanics, has seen tremendous advancement in the last few years, and is integrated into a textbook for the first time. These latter chapters emphasize rates of rare events like chemical reactions as well as single molecule experiments. Throughout the book, distinctions between heat and work, as well as notions of trajectory ensembles reflect the incorporation of stochastic thermodynamics into the modern language of statistical mechanics. Ideas of scaling, the concentration of measures, and generalized theories of ensemble equivalence represent the important contribution of the mathematics of large deviations.