  In this short review the results of detailed studies for dilepton production
from $p+A$ and $A+A$ reactions at SIS energies are presented. The calculations
are based on a semi-classical BUU transport model that includes the off-shell
propagation of vector mesons and evaluates the width of the vector mesons
dynamically. Different scenarios of in-medium modifications of vector mesons,
such as collisional broadening and dropping vector meson masses, are
investigated and the possibilities for an experimental observation of in-medium
effects in $p+A$ reactions at 1--4 GeV are discussed for a variety of nuclear
targets.
