Dilepton spectroscopy in heavy ion collisions is one of the most promising tools to investigate hadron properties in hot and dense nuclear matter. Spectral functions of light vector mesons, in particular short lived \rho , are the key observables to study such effects. The connection of the mesons to virtual radiation from hot and dense nuclear matter is emphasized. Recent experimental results on dilepton production in relativistic heavy ion collisions obtained by various experiments at RHIC, SPS and low energy SIS/Bevelac facilities are presented and discussed.

PACS numbers: 12.40.Vv, 21.65 Jk, 25.75.Nq, 24.85.+p