The fourth edition of "Quantum Chemistry" is an updated textbook on the subject covering the model syllabi of various undergraduate and postgraduate courses. The book contains the basics of quantum mechanics and quantum mechanical laws; applications of translational, vibrational and rotational motions of sub-atomic particles; theories of harmonic oscillator and atomic structure etc. The Hartree Fock self-consistent field methods, configuration, interaction, extended Huckel theory etc. are all presented with utmost clarity and examples. The present edition contains a chapter on matrix-vector methods of quantum mechanics as well as one on density functional theory along with molecular symmetry and group theory with applications to molecular orbital treatment. Steps involved in mathematical derivations are presented in full, leaving no ambiguity. Illustrative examples and practice problems, with hints are provided in each chapter.