  An essential prerequisite for quark-gluon plasma production in nuclear
collisions is cross-talk between the partons from different nucleons in the
colliding nuclei. The initial density of partons is determined by the parton
distribution functions obtained from deep inelastic lepton-hadron scattering
and by the nuclear geometry; it increases with increasing $A$ and/or $\sqrt s$.
In the transverse collision plane, this results in clusters of overlapping
partons, and at some critical density, the cluster size suddenly reaches the
size of the system. The onset of large-scale cross-talk through color
connection thus occurs as geometric critical behavior. Percolation theory
specifies the details of this transition, which leads to the formation of a
condensate of deconfined partons. Given sufficient time, this condensate could
eventually thermalize. However, already the onset of parton condensation in the
initial state, without subsequent thermalization, leads to a number of
interesting observable consequences.
