  The current theoretical status in the analysis and interpretation of low-mass
dilepton measurements in (ultra-) relativistic heavy-ion experiments is
reviewed. Special emphasis is put on potential signals of (partial) restoration
of dynamically broken chiral symmetry in a hot and dense hadronic medium. It
follows from chiral symmetry alone that parity partners of hadronic correlation
functions must become identical when the symmetry is restored. The assessment
of medium effects in the vector channel, which governs the dilepton production,
thus necessitates a simultaneous treatment of the vector and axialvector
degrees of freedom. While significant progress in this respect has been made
some open questions remain in establishing a rigorous link in the mass region
below 1 GeV. From the present calculations a suggestive 'quark-hadron duality'
emerges near the phase boundary. It implies substantial medium effects in the
dilepton signal from the hadronic phase which smoothly matches a perturbative
description within the plasma phase.
