Relativistic Nuclear Collisions and the QCD Phase Boundary

In nucleus-nucleus collisions at ultra-relativistic energies matter is formed with initial energy density significantly exceeding the critical energy density for the transition from hadronic to partonic matter. We will review the experimental evidence for this new form of matter - the Quark-Gluon Plasma - from recent experiments at the SPS and RHIC with emphasis on collective behavior, thermalization, and its opacity for fast partons. We will further show that one can determine from the data a fundamental QCD parameter, the critical temperature for the QCD phase transition.