This vibrant book takes the reader on an exciting journey through much of physical sciences by explaining the wide ranging influence of a single equation, the Boltzmann factor. This simple relation provides the key to understanding a remarkable variety of phenomena ranging from, for example, the behavior of atmospheric pressure at altitude, the appearance of molecular spectra and the factors that determine chemical equilibrium. Compact and clearly written book, this book, which requires no advanced mathematical skills, will enrich and deepen students' understanding of the molecular sciences. The insight it provides will enable them to appreciate the elegant unity that underlies the subject.