  Physics of charmonium suppression in nuclear collisions drastically changes
between the energies of SPS and RHIC. Mechanisms suppressing charmonia at 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, become a
dominant effect at RHIC providing a much stronger suppression. A onset of
coherence at Fermilab energies explains the observed cross section ratio
steeply falling at large Feynman x_F. In nuclear collisions variation of
charmonium suppression with x_F suggests a sensitive probe for QGP.
