Statistical thermodynamics plays a vital linking role between quantum theory and chemical thermodynamics, yet students often find the subject unpalatable. In this revised edition, the authors overcome this by emphasizing the concepts involved (particularly the significance of the partition function), rather than dwelling on the complex mathematical niceties that are essential for a profound study of the subject. Strong emphasis is placed on the physical basis of statistical thermodynamics. After a clear exposition of the distribution laws, partition functions, heat capacities, chemical equilibria, and kinetics, the subject is further illuminated by discussions of low-temperature effects, spectroscopy, and a variety of macroscopic behaviour of atoms and molecules.