This concise, elementary treatment illustrates the ways in which an atomic-molecular perspective yields new insights and powers operative in the realms of macroscopic thermodynamics. Starting with an analysis of some very simple microcanonical ensembles, it proceeds to the Boltzmann distribution law and a systematic exploration of the proper formulation, evaluation, and application of partition functions. The concepts of equilibrium and entropy thus acquire new significance, and readers discover how thermodynamic parameters may be calculated from spectroscopic data. Encompassing virtually all of the forms of statistical mechanics customary to undergraduate physical chemistry books, this brief text requires prior acquaintance with only the rudiments of the calculus and a few of the simplest propositions of classical thermodynamics. Appropriate for introductory college chemistry courses, it further lends itself to use as a supplementary text for independent study by more advanced students.