20 years of J/ $\psi$ suppression at the CERN SPS - Results from experiments NA38, NA51 and NA50

J/ $\psi$ suppression was predicted to be a signature of deconfinement and has been extensively studied at the CERN-SPS in proton, light and heavy ion induced reactions. Indeed, proton, O, S and Pb ion beams, at various incident energies, have been used at CERN on different targets, in order to precisely quantify J/ $\psi$ production as a function of both the colliding nuclei and the centrality of the collision itself. This paper reviews the results obtained so far with a critical discussion of the experimental conditions and successive improvements implemented for the various data samples. It makes a tentative summary of what has been learned on the subject, from the experimental point of view, in the SPS energy range