Deconfinement in QCD and in Nuclear Collisions

After summarizing the study of the states of matter in QCD, I address the efforts to create the predicted quark-gluon plasma with the help of high energy nuclear collisions and survey possible probes to investigate the produced medium. I then consider in some detail the partonic initial state provided by A - A collisions and define the necessary conditions for the formation of a large-scale interacting system. Finally I discuss to what extent some present data reflect geometric critical behavior in the initial state or thermal critical behavior in the final state.