Entropy of the QCD plasma

Self-consistent approximations in terms of fully dressed propagators provid e a simple expression for the entropy of an ultrarelativistic plasma, which isolates the contribution of the elementary excitations as a leading contr ibution. Further approximations, whose validity is checked on a soluble mod el involving a scalar field, allow us to calculate the entropy of the QCD p lasma. We obtain an accurate description of lattice data for purely gluonic QCD, down to temperatures of about twice the transition temperature.