  We calculate the quark contribution to the free energy of a hot quark-gluon
plasma to two-loop order using hard-thermal-loop (HTL) perturbation theory. All
ultraviolet divergences can be absorbed into renormalizations of the vacuum
energy and the HTL quark and gluon mass parameters. The quark and gluon HTL
mass parameters are determined self-consistently by a variational prescription.
Combining the quark contribution with the two-loop HTL perturbation theory free
energy for pure-glue we obtain the total two-loop QCD free energy. Comparisons
are made with lattice estimates of the free energy for N_f=2 and with exact
numerical results obtained in the large-N_f limit.
