  The NA60 experiment has studied low-mass muon pairs in 158 AGeV Indium-Indium
collisions at the CERN SPS. A strong excess of pairs is observed above the
expectation from neutral meson decays. The unprecedented sample size of 360 000
events and the good mass resolution of about 2% allow to isolate the excess by
subtraction of the known sources. The shape of the resulting mass spectrum is
consistent with a dominant contribution from pi+ pi- -> rho -> mu+ mu-
annihilation. The associated rho spectral function shows a strong broadening,
but essentially no shift in mass.
