Intended for undergraduate and postgraduate students of physics and chemistry, this textbook covers the entire spectrum of both non-relativistic quantum mechanics and relativistic quantum mechanics in a simple style to be truly beneficial to all levels of students. The book is organized in three parts. The first part describes the development of quantum theory, starting from Planck's law to de Broglie hypothesis. The Schrödinger theory has been introduced with relevant logic. This part also includes the treatments of angular momentum, path integral formalism, two-electron systems and interaction of radiation with matter. The second part deals with relativistic quantum theory including Klein–Gordon and Dirac equations, Weyl's neutrino theory, ending with gauge theory applied to Salam–Weinberg electroweak theory. The third part delves into the conceptual foundations of quantum mechanics, and Einstein–Bohr debate culminating in Bell's inequality theorem. The book concludes with a discussion on three quantum interference effects in electromagnetic and gravitation fields each of which has a deep physical importance. NEW TO THE SECOND EDITION • New topics on: o Bose Statistics o Periodic Potential o Clebsech – Gordan Coefficients o Quantum Entanglement • New Appendices on: o Noether's Theorem o Space Reflection, Time Reversal and Charge Conjugation TARGET AUDIENCE • B.Sc./M.Sc. (Physics) • B.Sc./M.Sc. (Chemistry)