This book and its prequel (Theories of Matter, Space, and Time: Classical Theories) grew out of courses that are taught by the authors on the undergraduate degree program in physics at Southampton University, UK. The authors aim to guide the full MPhys undergraduate cohort through some of the trickier areas of theoretical physics that undergraduates are expected to master. To move beyond the initial courses in classical mechanics, special relativity, electromagnetism and quantum theory to more sophisticated views of these subjects and their interdependence. This approach keeps the analysis as concise and physical as possible whilst revealing the key elegance in each subject discussed.This second book of the pair looks at ideas to the arena of Quantum Mechanics. First quickly reviewing the basics of quantum mechanics which should be familiar to the reader from a first course, it then links the Schrodinger equation to the Principle of Least Action introducing Feynman's path integral methods. Next, it presents the relativistic wave equations of Klein, Gordon and Dirac. Finally, Maxwell's equations of electromagnetism are converted to a wave equation for photons and make contact with Quantum Electrodynamics (QED) at a first quantized level. Between the two volumes the authors hope to move a student's understanding from their first courses to a place where they are ready to embark on graduate level courses on quantum field theory.