We present results from lattice calculations on the thermodynamics of QCD at
non-zero temperature and baryon chemical potential and discuss the role of
resonances for the occurrence of the transition to the quark-gluon plasma in
hot and dense matter. Properties of a hadronic resonance gas are compared to
lattice results on the equation of state at zero as well as non-zero baryon
chemical potential. Furthermore, it is shown that the quark mass dependence of
the transition temperature can be understood in terms of lines of constant
energy density in a resonance gas.Comment: This paper is based on talks given in July 2003 at the Workshop on
Finite Density QCD at Nara, Nara, Japan, and the International Symposium on
Color Confinement and Hadrons in Quantum Chromodynamics - Confinement 2003,
Wako, Japa
