  We briefly review the dilepton experiments at BEVALAC/SIS and SPS energies
for p+p, p+A and A+A collisions as well as our present understanding of the
data within transport theoretical simulations. Since dileptons from p+A and A+A
collisions in particular probe the in-medium spectral functions of vector
mesons, a novel semiclassical off-shell transport approach is introduced on the
basis of the Kadanoff-Baym equations that describes the dynamical evolution of
broad hadron spectral functions. The implications of the off-shell dynamics --
relative to the conventional on-shell transport dynamics -- are discussed for
proton spectra, high energy $\gamma$, pion, kaon and antikaon production from
GANIL to AGS energies in comparison to experimental data.
