  We develop the in-out formalism for one-loop effective actions in
electromagnetic fields in the space-dependent gauge. We further advance a
method using the inverse scattering matrix to calculate the effective actions
in pure magnetic fields, find the effective actions in a constant magnetic
field and a localized Sauter-type magnetic field and apply the uniform
semiclassical approximation to the effective action in a general magnetic
field. In the in-out formalism we show that the one-loop effective actions in
constant fields and Sauter-type fields exhibit the electromagnetic duality.
