Many facets of quantum chromodynamics (QCD) are relevant to the in-depth discussion of theoretical and experimental aspects of high-energy nucleus-nucleus collisions. Exciting phenomena are being discovered in such ultrarelativistic heavy ion collisions, notably the increasingly important role of deconfined quark-gluon matter created in the early stage. The book contains lectures on the physics of hot dense matter, the expected phase transitions and colour superconductivity, recent developments in the treatment of nonlinear effects at large parton densities, fundamental issues in the phenomenology of ultrarelativistic heavy collisions. The latest data on heavy ion collisions are also presented.