This book is intended for the undergraduate level physical chemistry course and is typically taught in the junior or senior year. This book is well-regarded for its unified treatment of macroscopic and molecular level phenomena and the interrelationship between the two. In this edition, Barrow has extensively revised the thermodynamics, quantum mechanics and spectroscopy chapters. The book provides a sound but manageable guide that takes readers into the subject, in spite of the generally modest background they have on the subject matter. The physical basis of essential ideas is emphasized and mathematical developments are simple and closely tied to the physical bases. The overall organization is such that topics are introduced in stages so students can become familiar with, and grow into each topic, and chapters are written so that there is a flexibility in the order in which they may be used.