The NA60 experiment at the CERN SPS has measured muon pairs with unprecedented precision in 158  A  GeV In–In collisions. A strong excess of pairs above the known sources is observed in the whole mass region 0.2< M <2.6 GeV. The mass spectrum for M <1 GeV is consistent with a dominant contribution from π + π − → ρ → μ + μ − annihilation. The associated ρ spectral function shows a strong broadening, but essentially no shift in mass. For M >1 GeV, the excess is found to be prompt, not due to enhanced charm production, with pronounced differences to Drell–Yan pairs. The slope parameter T eff associated with the transverse momentum spectra rises with mass up to the ρ , followed by a sudden decline above. The rise for M <1 GeV is consistent with radial flow of a hadronic emission source. The seeming absence of significant flow for M >1 GeV and its relation to parton–hadron duality is discussed in detail, suggesting a dominantly partonic emission source in this region. A comparison of the data to the present status of theoretical modeling is also contained. The accumulated empirical evidence, including also a Planck-like shape of the mass spectra at low p T and the lack of polarization, is consistent with a global interpretation of the excess dimuons as thermal radiation. We conclude with first results on ω in-medium effects.