  We present lattice QCD results on heavy quark free energies, extract from its
temperature dependence entropy and internal energy contributions, and discuss
the onset of medium effects that lead to screening of static quark-antiquark
sources in a thermal medium. Most results are obtained in (2+1)-flavour QCD on
a line of constant physics with almost realistic quark masses and compared to
previous results from 2-flavor QCD as well as pure gauge theory. Furthermore,
we discuss results on the density dependence of screening masses that have been
obtained using a leading order Taylor expansion in the baryon chemical
potential.
