$J/\psi$ and $\psi$' suppression in nucleus-nucleus interactions

We report on the production of high mass muon pairs in $p$ - A, S - U and Pb - Pb collisions, as measured by the NA38 and NA50 experiments at the CERN SPS. The ratio between $\psi'$ and $J/\psi$ cross-sections is found to be constant in $p$ - A interactions but decreases from $p$ - A to S - U and as the released transverse energy, $E_{T}$, increases. The first preliminary results on Pb - Pb collisions, obtained by the NA50 experiment (the successor of NA38), indicate that the $J/\psi$ production is suppressed relative to the Drell-Yan continuum, from peripheral to central collisions, as previously observed in O - U and S - U interactions.