RHIC (Brookhaven) and ALICE (CERN) have renewed interest in QCD at high temperature and density, where the quark gluon plasma shows signs of being a perfect fluid. There is also interest in the idea that cool and dense QCD (as in neutron stars) may be a colour superconductor. We study these phases using thermal models and through weakly coupled string theory duals to strongly coupled gauge theories. The latter, using anti-de-Sitter black holes and their quasi-normal modes, allow the computation of transport coefficients in the plasma as well as computing properties of mesons melting in the background thermal bath.