Transition from nuclear matter to quark matter

High energy heavy ion collisions which offer the unique possibility of observing the formation of quark matter are discussed. Thermalization is possible if the particles scatter off each other very often and for this to happen they must be produced with a certain transverse momentum. It is necessary to accelerate heavy ions to reach the phase transition point. Heavy lepton pairs escape from the plasma immediately after formation and thus give information about the parameters of the plasma. (M.F.W.)