  We confront lattice QCD results on the transition from the hadronic phase to
the quark--gluon plasma with hadron resonance gas and percolation models. We
argue that for T < T_c the equation of state derived from Monte--Carlo
simulations of (2+1) quark--flavor QCD can be well described by a hadron
resonance gas. We examine the quark mass dependence of the hadron spectrum on
the lattice and discuss its description in terms of the MIT bag model. This is
used to formulate a resonance gas model for arbitrary quark masses which can be
compared to lattice calculations. We finally apply this model to analyze the
quark mass dependence of the critical temperature obtained in lattice
calculations. We show that the value of T_c for different quark masses agrees
with lines of constant energy density in a hadron resonance gas. For large
quark masses a corresponding contribution from a glueball resonance gas is
required.
