"Relativistic Quantum Mechanics and Quantum Field Theory" deals with the single particle relativistic wave equations and the elements of quantum field theory. The Klein-Gordon equation is discussed briefly and elaborately the Dirac equation, its free particle solutions and Feynman's positron theory with a view to emphasize how the intuitive approach of Feynman has enormously simplified the calculations in Quantum Electrodynamics - the interaction of radiation with matter - by the introduction of Feynman diagrams. It is shown that Feynman's approach is equivalent to the other more general approach of quantum field theory and the equivalence of the two approaches has been demonstrated through the S-matrix formalism which leads to the Feynman diagrams by the application of Wick's theorem. How the quantum field theory is stretched beyond quantum electrodynamics to include electro weak interactions and strong interactions and how it leads to the formulation of the standard model of elementary particles are briefly discussed.