  Medium Effects in low-mass dilepton production in ultra-relativistic
heavy-ion collisions are investigated using hadronic models. The rescattering
of pions and rho mesons within a hot and dense hadron gas leads to substantial
modifications in the $\pi^+\pi^- \to e^+ e^-$ annihilation process, which are
shown to be important for understanding the recently observed dilepton
enhancement at the CERN-SpS. Possible implications for the nature of the chiral
phase transition are outlined.
