I consider teaching to have been one of my greatest privileges in life. I have always attemped to make a matter, sometimes difficult, sometimes in constant evolution, simpler and easier to understand. There comes a time then, when one feels the necessity to write and publish a book. For this reason, I have already published (in french) several lecture books on Classical Mechanics, Quantum Mechanics, Matrix Optics, Electroweak Interaction, General Rel­ ativity and Gravitation, Cosmology. Teaching quantum theory has been a particular delight as its constant evolution and enlargement, embraces all domains of physics. Even if the general layout remains relatively unchanged, the evolution of our understanding of the physical world imposes the in­ troduction of new approaches. It therefore seemed that a physics textbook, even one with a graduate readership in mind, had to introduce the Dirac Electron Theory and some rudimentary material on quantum field theory. This holds for also for explanations of the spontaneous symmetry breaking of Higgs scalar fields that gives mass to the bosons involved in short range interactions. We have deliberately cut down on material on some "classical" topics of quantum theory to make space for less known methods. Examples of the latter include the Feynman path integral "3rd quantization" and the interpretation of quantum mechanics in terms of phase focusing coherence.