  The Schwinger equations of QED are rewritten in three different ways as
integral equations involving functional derivatives, which are called weak
field, strong field, and SCF quantum electrodynamics. The perturbative
solutions of these equations are given in terms of appropriate Feynman
diagrams. The Green function that is used as an electron propagator in each
case is discussed in detail. The general renormalization rules for each of the
three equations are provided both in a non perturbative way (Dyson relations)
and for Feynman diagrams.
