The spectrum of hadrons is intimately related to the structure of the QCD vacuum, which is filled with quark and gluon condensates. Thus, upon heating the vacuum, a melting of the condensates is bound to induce medium modifications of the hadronic spectrum, ultimately leading to the formation of the quark-gluon plasma. Dilepton invariant-mass spectra are the only known observable in high-energy heavy-ion collisions which enable a direct access to medium modifications of a hadronic spectral function (in the vector channel). In particular, thermal radiation of low-mass dileptons (M<1.5GeV) has long been regarded as the most promising tool to ``detect" the melting of the chiral quark condensate (aka ``chiral symmetry restoration"), through the modifications of the light vector mesons rho, omega, phi. We discuss recent progress in this endeavor, in particular by using sum rules to connect order parameters from numerical lattice-QCD computations with effective hadronic theory, and the interpretation of dilepton data including results from the beam energy scan program at RHIC.