Over the past 15 years, an extensive program of high energy nuclear collisi
ons at BNL and CERN has been devoted to the experimental search for the qua
rk-gluon plasma predicted by QCD. The start of RHIC this year will increase
the highest available collision energy by a factor 10. This seems a good t
ime for a critical assessment: what have we learned so far and what can we
hope to learn in the coming years?