Theory and experiment in chemistry today provide a wealth of data, but such data have no meaning unless they are correctly interpreted by sound and transparent physical models. Linus Pauling was a grandmaster in the modelling of molecular properties. Indeed, many of his models have served chemistry for decades and that has been his lasting legacy for chemists all over the world.The aim of this book is to put such simple models into the language of modern quantum chemistry, thus providing a deeper justification for many of Pauling’s ideas and concepts. However, it should be stressed that many contributions to this work, written by some of the world’s most prominent theoretical chemists, do not merely follow Pauling’s footprints. By taking his example, they made bold leaps forward to overcome the limitations of the old models, thereby opening new scientific vistas.This book is an important contribution to the chemical literature. It is an almost obligatory textbook for postgraduate students and postdoctoral researchers in physical chemistry, chemical physics and advanced physical organic chemistry.