The present book entitled A Handbook of Quantum Mechanics and Spectroscopy covers most recent developments in these aspects of Physical Chemistry. An attempt has been made in this volume to present the complicated subject matter in a simple and easy to understand manner. To make clear the different aspects and for comparative studies the book has been divided into seven chapters. An attempt has been made to keep each chapter self-sufficient and to avoid cross references so that the reader may be able to satisfy all his curiosities without much difficulty. Data presented in the book have been taken from authentic sources and can be relied upon by research workers engaged in the field.