Hard probes in nucleus-nucleus collisions

The present knowledge on hard processes in the context of heavy ion collisions is reviewed, with particular emphasis on J/ψ production. The p-A data on charmonia production from Fermilab experiments is shown to be in excellent agreement with the p-A data collected at CERN. The simultaneous analysis of all existing p-A data reaches a precision which allows us to rule out some preconceived ideas, setting a good frame against which the data collected with ion beams at CERN can be compared. (orig.)