Thermal Hadronic Properties and QCD Phase Transitions

After a general introduction to the Lagrangian of QCD (Quantum Chromodynamics) and its symmetries, I will present the QCD Sum Rules approach for studying hadronic properties. This will be generalized to a finite temperature scenario, where we expect that phase transitions like deconfinement and/or chiral symmetry restorations should occur. In particular we will present our results for the rho meson spectrum, reconstructed from the dimuon spectrum in heavy ion collisions, and for charmonium resonances which could survive beyond the critical temperature. I will try to avoid technical details, emphasizing the physical and general aspects of our approach.