Heavy ion collisions

Heavy ion collisions at ultra-relativistic energies offer the unique possibility to study strongly interacting matter under extreme conditions of temperature and density and to search for the conjectured quark-gluon-plasma. The first phase of experiments with relatively light ions is now completed and a new round of much heavier ions has started with 10 GeV/u Au beams at BNL and 160 GeV/u Pb beams at CERN. A general overview of the present experimental status is given emphasizing those results which clearly show that heavy ion collisions cannot be interpreted as a mere superposition of independent nucleon-nucleon collisions.