Matter in thermal equilibrium radiates photons, whose spectrum reflects the temperature as well as possible critical behavior in the vicinity of phase transitions. They are ideal probes for the elementary excitations in a given system. Virtual photons, the generalized form of electromagnetic radiation, materialize after short time by formation of a pair of charged leptons (di-leptons). The spectra of di-leptons have long been recognized as a unique observable to probe the interior of the fireball. Unlike hadrons, di-leptons do not interact strongly. Once produced, they escape the fireball undistorted, thus retain the information imprinted on them at the time of their creation.
In this contribution, I will first recall the basic tools and ideas of dilepton spectroscopy in heavy-ion collisions and discuss recent highlights of extracting properties of the QCD medium, as well as diagnosing excitation functions of fireball bulk properties. Special attention will be given to the effects of high baryon densities and the opportunities that arise from these in heavy-ion collisions in GSI/FAIR energy regime.