  We present a unified description of the vector meson and dilepton production
in elementary and in heavy ion reactions. The production of vector mesons
($\rho,\omega,\phi$) is described via the excitation of nucleon resonances
($R$). The theoretical framework is an extended vector meson dominance model
(eVMD) for resonance decays $R\longmapsto NV$ with arbitrary spin which is
covariant and kinematically complete. The eVMD includes thereby excited vector
meson states in the transition form factors. The model has successfully been
applied to $\omega$ and $\phi$ production in $p+p$ reactions. The same model is
used to describe the dilepton production in elementary reactions where
corresponding data are well reproduced. However, when the model is applied to
heavy ion reactions in the BEVALAC/SIS energy range the experimental dilepton
spectra measured by the DLS Collaboration are significantly underestimated at
small invariant masses. In view of this fact we discuss further medium effects:
One is a substantial collisional broadening of the $\rho$ and in particular of
the $\omega$ meson in the vicinity of the $\rho/\omega$-peak. The second medium
effect is the destruction of quantum interference in a dense medium. A
decoherent dilepton emission through vector mesons decays enhances the
corresponding low mass dilepton yield in heavy ion reactions and improves the
agreement with existing data.

Nuclear Theory