  We analyze the quark mass and flavour dependence of the QCD phase transition
temperature. When the lightest pseudo-scalar meson mass (m_PS) is larger than 2
GeV the critical temperature is controlled by the gluonic sector of QCD alone.
For smaller values of the lightest meson mass the pseudo-critical temperature
decreases slowly with m_PS. For a large regime of meson masses the
pseudo-critical temperature of 2-flavour QCD is about 10% larger than in the
3-flavour case. On lattices with temporal extent N_t=4 an extrapolation to the
chiral limit yields T_c = 173(8) MeV and 154(8) MeV for 2 and 3-flavour QCD,
respectively. We also analyze dynamical quark mass effects on the screening of
the heavy quark potential. A detailed analysis of the heavy quark free energy
in 3-flavour QCD shows that close to T_c screening effects are approximately
quark mass independent already for pseudo-scalar meson masses m_PS = 800 MeV
and screening sets in at distances r = 0.3 fm.
