2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/148051We give a review of modern theoretical understanding of the problem of thermal phase transition in $QCD$. The existence or non-existence of such transition depends on the nature of the gauge group, the number of light quark flavors , and on the value of quark masses. Numerical lattice measurements indicate that the phase transition {\it does} not occur for the experimentally observed values of quark masses.Lecture at the International School of Physics "Enrico Fermi", Varenna, July, 1995. 19 pages LaTeX, 3 ps files for figures in a separate uu-file. Minor stylistic revisionsHigh Energy Physics - Phenomenologytext