  We begin by reviewing the derivation of generalized Maxwell equations from an
operational definition of the electromagnetic field and the most basic notions
of what constitutes a dynamical field theory. These equations encompass the
familiar Maxwell equations as a special case but, in other cases, can predict
birefringence, charge non-conservation, wave damping and other effects that the
familiar Maxwell equations do not. It follows that observational constraints on
such effects can restrict the dynamics of the electromagnetic field to be very
like the familiar Maxwellian dynamics, thus, providing an empirical foundation
for the Maxwell equations. We discuss some specific observational results that
contribute to that foundation.
