  In relativistic heavy ion collisions, a highly occupied gluonic matter is
created shortly after initial impact, which is in a non-thermal state and often
referred to as the Glasma. Successful phenomenology suggests that the glasma
evolves rather quickly toward the thermal quark-gluon plasma and a hydrodynamic
behavior emerges at very early time $\sim \hat{o}(1)$ fm/c. Exactly how such
"apparent thermalization" occurs and connects the initial conditions to the
hydrodynamic onset, remains a significant challenge for theory as well as
phenomenology. We briefly review various ideas and recent progress in
understanding the approach of the glasma to the thermalized quark-gluon plasma,
with an emphasis on the kinetic theory description for the evolution of such
far-from-equilibrium and highly overpopulated, thus weakly-coupled yet strongly
interacting glasma.
