The physics of charmonium suppression in nuclear collisions drastically changes between the energies of SPS and RHIC. Mechanisms suppressing charmonia at the SPS are reviewed, neither of which is important at RHIC. On the other hand, coherence, or shadowing of c quarks and gluons barely seen at SPS, becomes a dominant effect at RHIC providing a much stronger suppression. The onset of coherence at Fermilab energies explains why the observed cross section ratio falls steeply at large Feynman x(F). In nuclear collisions the variation of charmonium suppression with x(F) suggests a sensitive probe to look for the QGP. [References: 20]
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