The study of topology is supposed to have started with a problem concerning the seven bridges of Koenigsberg ; the study of ergodic theory originated with certain considerations in statistical mechanics. The mathematical outgrowth of the bridge problem is a theorem about odd and even vertices in graphs; the mathematical outgrowth of the gas problem is a theorem about the asymptotic behavior of measure-preserving transformations. In both cases the direct outgrowth of the original motivation is only a small part of an extensive theory. Since, nevertheless, there is some value in understanding the historical background of a theory, I shall spend the first few minutes of these lectures on a crude description of the relevant part of statistical mechanics.