Hagedorn's Hadron Mass Spectrum and the Onset of Deconfinement

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.