  We discuss experimental data on particle yields and particle spectra obtained
in heavy ion collisions in a very broad energy range from SIS/GSI through
AGS/BNL up to SPS/CERN and RHIC/BNL. We argue that in this broad energy range
hadronic yields and their ratios resemble a thermal equilibrium population
along a unified freeze--out curve determined by the condition of fixed
energy/particle = 1 GeV. At RHIC and top SPS, thermal parameters are consistent
within error with the critical conditions required for deconfinement. This,
together with the particular distribution of strangeness within a collision
fireball, could indicate that chemical equilibrium is a direct consequence of
parton to hadron transition, which populates a state of maximum entropy. At
lower energies equilibration in A--A collisions should appear through hadronic
interactions and rescatterings.
