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