The book contains the developments in quantum mechanics as well as the basic concepts of quantum formalism in simple terms. Quantum Mechanics is the science of motion of atomic and sub atomic Particles. Experimental measurements for atomic and molecular systems shows that an electron moving around the nucleus of an atom has only a discrete set of values of energy. Rotational motion of particle is of great interest in atomic and molecular problems. For single particle executing such a motion, the Schrödinger equation can be solved. Graduate and senior undergraduate students in Physics, or engineering students, who intend to do research in Physics should find this book useful.