  We investigate dilepton production in transport-based approaches and show
that the baryon couplings of the $\rho$ meson represent the most important
ingredient for understanding the measured dilepton spectra. At SIS energies,
the baryon resonances naturally play a major role and affect already the vacuum
spectra via Dalitz-like contributions, which can be captured well in transport
simulations. Recent pion-beam measurements at GSI will help to constrain the
properties of the involved resonances further.
