  We review the current understanding of time-like virtual photon emission from
QCD matter. The phenomenology of dilepton emission is discussed and basic
theoretical concepts are introduced. The experimental findings are presented,
grouped into production of lepton pairs in elementary processes, production off
cold nuclear matter and emission from heavy-ion collisions. The review
emphasizes the role of dilepton emission as tool for studying exotic phases of
QCD matter. Open questions and a route to probe the QCD phase diagram with
dileptons are outlined.
