This subject is developed for readers with a fairly good knowledge of classical mechanics, electrodynamics and theory of relativity. The mathematics of linear vector spaces and matrices required is included as text and appendices. All principles and techniques are explained with illustrative applications. A Hilbert space formulation of the basic principles and the equations of motion are adopted at the outset. The treatment of linear vector spaces, matrices, angular momentum, relativistic wave equations, quantum field theory and the interpretational problem, is given in a more detailed way than in most books on quantum mechanics. Topics covered include Clebsch-Gordon and Racah coefficients, 9-j symbols and spherical tensors, the Klein-Gordon and the Weyl equations, Feynman's path-integral formalism, Feynman diagrams, Normal Products and Wick's Theorem, the EPR paradox, Hidden-variables theories and Bell's inequality. A number of problems are included with a view to supplementing the text. The present edition includes boundary value problems, representation theory, Fermi-Gas Model of the nuclei, Imaginary-mass Klein-Gordon equation, covariant and contravariant vectors, explanations of the EPR paradox and Einstein's concept of locality versus determinism.