In this unique textbook Jack Simons goes back to basics and focuses on the foundations that lie at the heart of modern day theoretical chemistry. Emphasis is on the concepts, tools and equations that govern the three main theoretical chemistry sub-disciplines: electronic structure, statistical mechanics and reaction dynamics. Part I provides the foundations of quantum mechanics and molecular spectroscopy as applied to chemistry today. This section can be used either as stand-alone material in a junior level physical chemistry class or to provide the reader with the tools and background needed to cover the second part of the book. Part II starts with a general overview of theoretical chemistry and then gives a very accessible introduction to each of the three main sub-disciplines in the subject. Highly illustrated with numerous exercises and worked solutions, this book provides a concise, up-to-date treatise on the underpinnings of modern theoretical chemistry.