  Starting from the earlier notions of stationary action principles, we show
how Julian Schwinger's Quantum Action Principle descended from Dirac's
formulation, which independently led Feynman to his path-integral formulation
of quantum mechanics. The connection between the two is brought out, and
applications are discussed. The Keldysh-Schwinger time-cycle method of
extracting matrix elements in nonequilibrium situations is described. The
variational formulation of quantum field theory and the development of source
theory constitute the latter part of this work. In this document, derived from
Schwinger's lectures over four decades, the continuity of concepts, such as
that of Green's functions, becomes apparent.
