Quarkonium Suppression from SPS to RHIC (and from p+A to A+A)

Heavy quarkonium production is expected to be sensitive to the formation of a quark gluon plasma (QGP). It was (and still is with ongoing data analyses) extensively studied at the CERN SPS, at collision energy sqrt(s_{N N}) of the order of 20 GeV. An anomalous suppression was clearly observed. The PHENIX experiment at RHIC has presented preliminary results that exhibit a similar amount of J / ψ suppression, at ten times higher collision energy. I review the results obtained at both facilities. While interpreting and comparing them, the importance of understanding normal nuclear effects is emphasized. A new method to derive a reference for Au+Au collisions from the centrality dependence of d+Au measurements at RHIC is presented.