Principles of Quantum Chemistry focuses on the application of quantum mechanics in physical models and experiments of chemical systems. This book describes chemical bonding and its two specific problems - bonding in complexes and in conjugated organic molecules. The very basic theory of spectroscopy is also considered. Other topics include the early development of quantum theory; particle-in-a-box; general formulation of the theory of quantum mechanics; and treatment of angular momentum in quantum mechanics. The examples of solutions of Schroedinger equations; approximation methods in quantum chemistry; symmetry in chemistry; and molecular-orbital theory are also covered. This publication is recommended for students taking undergraduate and graduate courses in quantum chemistry.