  Over the past 15 years, an extensive program of high energy nuclear
collisions at BNL and CERN has been devoted to the experimental search for the
quark-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 time
for a critical assessment: what have we learned so far and what can we hope to
learn in the coming years?
