This last volume of the Berkeley Physics Course is devoted to the study of large-scale systems consisting of many atoms or molecules:thus it provides an introduction to the subjects of statistical mechanics, kinetic theory, thermo- dynamics, and heat. The approach is not patterned upon the historical development of these subjects and does not pro- ceed along conventional lines. The aim has been to adopt a modern point of view and to show, in as systematic and simple way as possible, how the basic notions of atomic theory lead to a conceptual framework capable of describing and predicting the properties of macroscopic systems.