Twenty years ago, the seven experiments on CERN's Heavy Ion programme at the Super Proton Synchrotron
(NA44, NA45/CEREA,NA49, NA50,NA52/NEWMASS, WA97/NA57 and WA98) combining their data were able to
characterise a new state of matter, verifying an important prediction of the theory of fundamental forces between quarks.
This talk briefly reviews studies of the phase diagram of strongly interacting matter with relativistic nuclear collisions
at the CERN Super Proton Synchrotron which followed the observation of the quark-gluon plasma.
The first ten years of this period was mostly influenced by results of the NA49 experiment, the second period was dominated by NA61/SHINE.
I will close by suggesting priorities for future measurements.
M. Pepe-Altarelli, G. Unal