  A brief history of the observation of the onset of deconfinement - the
beginning of the creation of quark gluon plasma in nucleus-nucleus collisions
with increasing collision energy - is presented. It starts with the measurement
of hadron mass spectrum and the Hagedorn's hypothesis of the limiting
temperature of hadronic matter (the Hagedorn temperature). Then the conjecture
that the Hagedorn temperature is the phase transition temperature was
formulated with the crucial Hagedorn participation. It was confirmed by the
observation of the onset of deconfinement in lead-lead collisions at the CERN
SPS energies.
