This book presents active application aspects of theoretical chemistry, and is particularly intended for experimental chemists, ranging from graduate students to more professional researchers, who are developing new materials or searching for novel properties of the materials they work with. It not only addresses the fundamental aspects of theoretical chemistry but also provides abundant examples of applications based on the electronic structure analyses of actual systems. As the book demonstrates, these analyses can deepen our understanding of a variety of chemical phenomena, including the chemical reactivities and electronic properties of substances, in a bottom-up manner. By illustrating how electronic structure analyses can be effectively applied, the book introduces readers to the impressive potential of theoretical chemistry, which they can adapt for their own purposes, and without having to suffer through a parade of complex formulae.