  In this talk, I will summarize the present state of a long-term effort to
obtain information on the high-order asymptotic behaviour of the QED
perturbation series through the effective action. Starting with the
constant-field case, I will discuss the Euler-Heisenberg Lagrangian in various
dimensions, and up to the three-loop level. This Lagrangian holds the
information on the N-photon amplitudes in the low-energy limit, and combining
it with spinor helicity methods explicit all-N results can be obtained at the
one-loop and, for the "all + " amplitudes, also at the two-loop level. For the
imaginary part of the Euler-Heisenberg Lagrangian, an all-loop formula has been
conjectured independently by Affleck, Alvarez and Manton for Scalar QED, and by
Lebedev and Ritus for Spinor QED. This formula can be related through a Borel
dispersion relation to the leading large-N behaviour of the N-photon
amplitudes. It is analytic in the fine structure constant, which is puzzling
and suggests a diagrammatic investigation of the large-N limit in perturbation
theory. Preliminary results of such a study for the 1+1 dimensional case throw
doubt on the validity of the conjecture.
