A wealth of data on ultra-relativistic nucleus-nucleus collisions has
been obtained at the SPS in recent years in a search for the high-density deconfined state of matter, the quark gluon plasma. The evidence for its fleeting existence in the early stage of the reactions is reviewed from an experimentalist's point of view. A deconfined phase is required to provide a consistent explanation of the diverse results.
Proof of a fully developed quark gluon plasma stage needs the higher
collision energies of RHIC and LHC.