Schwingers Quantum Action Principle : From Dirac’s Formulation Through Feynman’s Path Integrals the Schwinger Keldysh Method Quantum Field Theory to Source Theory Starting from the earlier notions of stationary action principles these tutorial notes shows how Schwinger’s Quantum Action Principle descended from Dirac’s formulation which independently led Feynman to his path integral formulation of quantum mechanics. Part I brings out in more detail the connection between the two formulations and applications are discussed. Then the Keldysh Schwinger time cycle method of extracting matrix elements is described. Part II will discuss the variational formulation of quantum electrodynamics and the development of source theory