The Vector Probe in Heavy-Ion Reactions An assessment is given of our understanding of the vector probe of hot and dense matter. Its uniqueness resides on the fact that it couples to virtual and real photons, giving access to the study of light (rho, omega, phi) and heavy (J/Psi, Psi') vector mesons in medium, as well as QCD blackbody radiation. Relations to the chiral/deconfinement phase transition will be discussed with respect to observable signatures.