  We discuss under what conditions the duality between electric and magnetic
fields is a valid symmetry of macroscopic quantum electrodynamics. It is shown
that Maxwell's equations in the absence of free charges satisfy duality
invariance on an operator level, whereas this is not true for Lorentz forces
and atom--field couplings in general. We prove that derived quantities like
Casimir forces, local-field corrected decay rates as well as van-der-Waals
potentials are invariant with respect to a global exchange of electric and
magnetic quantities. This exact symmetry can be used to deduce the physics of
new configurations on the basis of already established ones.
