  A systematic analysis of data on strangeness and pion production in
nucleon-nucleon and central nucleus-nucleus collisions is presented. It is
shown that at all collision energies the pion/baryon and strangeness/pion
ratios indicate saturation with the size of the colliding nuclei. The energy
dependence of the saturation level suggests that the transition to the Quark
Gluon Plasma occurs between 15 AGeV/c (BNL AGS) and 160 AGeV/c (CERN SPS)
collision energies. The experimental results interpreted in the framework of a
statistical approach show that the effective number of degrees of freedom
increases in the course of the phase transition and that the plasma created at
CERN SPS energies may have a temperature of about 280 MeV (energy density 10
GeV/fm-3). The presence of the phase transition can lead to the non-monotonic
collision energy dependence of the strangeness/pion ratio. After an initial
increase the ratio should drop to the characteristic value for the QGP. Above
the transition region the ratio is expected to be collision energy independent.
Experimental studies of central Pb+Pb collisions in the energy range 20--160
AGeV/c are urgently needed in order to localize the threshold energy, and study
the properties of the QCD phase transition.
