In this unique textbook Jack Simons goes back to basics and focusses on the foundations that lie at the heart of modern theoretical chemistry. Emphasis is on the concepts, tools and equations that govern the three main theoretical chemistry sub-disciplines. An associated website supplies supplementary material plus a host of links to other theoretical chemistry sites. Ideal for advanced undergraduate or graduate students, this book is highly illustrated and contains over fifty multi-part exercises and worked solutions. It provides a concise, up-to-date treatise on the underpinnings of modern theoretical chemistry.